Remove wasm-builder's README (#2525)

Followup of https://github.com/paritytech/polkadot-sdk/pull/2217

This PR deletes the README of the `wasm-builder` crate and moves its
docs back into the rustdoc, [as requested
here](https://github.com/paritytech/polkadot-sdk/pull/2217#discussion_r1406401175).
(:

substrate/utils/wasm-builder/src/lib.rs

@@ -15,7 +15,102 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![doc = include_str!("../README.md")]
+//! # Wasm builder is a utility for building a project as a Wasm binary
+//!
+//! The Wasm builder is a tool that integrates the process of building the WASM binary of your
+//! project into the main `cargo` build process.
+//!
+//! ## Project setup
+//!
+//! A project that should be compiled as a Wasm binary needs to:
+//!
+//! 1. Add a `build.rs` file.
+//! 2. Add `wasm-builder` as dependency into `build-dependencies`.
+//!
+//! The `build.rs` file needs to contain the following code:
+//!
+//! ```no_run
+//! use substrate_wasm_builder::WasmBuilder;
+//!
+//! fn main() {
+//!     WasmBuilder::new()
+//!         // Tell the builder to build the project (crate) this `build.rs` is part of.
+//!         .with_current_project()
+//!         // Make sure to export the `heap_base` global, this is required by Substrate
+//!         .export_heap_base()
+//!         // Build the Wasm file so that it imports the memory (need to be provided by at instantiation)
+//!         .import_memory()
+//!         // Build it.
+//!         .build()
+//! }
+//! ```
+//!
+//! As the final step, you need to add the following to your project:
+//!
+//! ```ignore
+//! include!(concat!(env!("OUT_DIR"), "/wasm_binary.rs"));
+//! ```
+//!
+//! This will include the generated Wasm binary as two constants `WASM_BINARY` and
+//! `WASM_BINARY_BLOATY`. The former is a compact Wasm binary and the latter is the Wasm binary as
+//! being generated by the compiler. Both variables have `Option<&'static [u8]>` as type.
+//!
+//! ### Feature
+//!
+//! Wasm builder supports to enable cargo features while building the Wasm binary. By default it
+//! will enable all features in the wasm build that are enabled for the native build except the
+//! `default` and `std` features. Besides that, wasm builder supports the special `runtime-wasm`
+//! feature. This `runtime-wasm` feature will be enabled by the wasm builder when it compiles the
+//! Wasm binary. If this feature is not present, it will not be enabled.
+//!
+//! ## Environment variables
+//!
+//! By using environment variables, you can configure which Wasm binaries are built and how:
+//!
+//! - `SKIP_WASM_BUILD` - Skips building any Wasm binary. This is useful when only native should be
+//!   recompiled. If this is the first run and there doesn't exist a Wasm binary, this will set both
+//!   variables to `None`.
+//! - `WASM_BUILD_TYPE` - Sets the build type for building Wasm binaries. Supported values are
+//!   `release` or `debug`. By default the build type is equal to the build type used by the main
+//!   build.
+//! - `FORCE_WASM_BUILD` - Can be set to force a Wasm build. On subsequent calls the value of the
+//!   variable needs to change. As wasm-builder instructs `cargo` to watch for file changes this
+//!   environment variable should only be required in certain circumstances.
+//! - `WASM_BUILD_RUSTFLAGS` - Extend `RUSTFLAGS` given to `cargo build` while building the wasm
+//!   binary.
+//! - `WASM_BUILD_NO_COLOR` - Disable color output of the wasm build.
+//! - `WASM_TARGET_DIRECTORY` - Will copy any build Wasm binary to the given directory. The path
+//!   needs to be absolute.
+//! - `WASM_BUILD_TOOLCHAIN` - The toolchain that should be used to build the Wasm binaries. The
+//!   format needs to be the same as used by cargo, e.g. `nightly-2020-02-20`.
+//! - `WASM_BUILD_WORKSPACE_HINT` - Hint the workspace that is being built. This is normally not
+//!   required as we walk up from the target directory until we find a `Cargo.toml`. If the target
+//!   directory is changed for the build, this environment variable can be used to point to the
+//!   actual workspace.
+//! - `WASM_BUILD_STD` - Sets whether the Rust's standard library crates will also be built. This is
+//!   necessary to make sure the standard library crates only use the exact WASM feature set that
+//!   our executor supports. Enabled by default.
+//! - `CARGO_NET_OFFLINE` - If `true`, `--offline` will be passed to all processes launched to
+//!   prevent network access. Useful in offline environments.
+//!
+//! Each project can be skipped individually by using the environment variable
+//! `SKIP_PROJECT_NAME_WASM_BUILD`. Where `PROJECT_NAME` needs to be replaced by the name of the
+//! cargo project, e.g. `kitchensink-runtime` will be `NODE_RUNTIME`.
+//!
+//! ## Prerequisites:
+//!
+//! Wasm builder requires the following prerequisites for building the Wasm binary:
+//!
+//! - rust nightly + `wasm32-unknown-unknown` toolchain
+//!
+//! or
+//!
+//! - rust stable and version at least 1.68.0 + `wasm32-unknown-unknown` toolchain
+//!
+//! If a specific rust is installed with `rustup`, it is important that the wasm target is
+//! installed as well. For example if installing the rust from 20.02.2020 using `rustup
+//! install nightly-2020-02-20`, the wasm target needs to be installed as well `rustup target add
+//! wasm32-unknown-unknown --toolchain nightly-2020-02-20`.
 
 use std::{
 	env, fs,