update contributing guide and ui-tests scripts (#1668)

- Updated ./docs/CONTRIBUTION.md to clarify how to use command bot and
update ui-tests locally or in PR
- Moved update-ui-tests.sh to a root of project to run them along for
substrate and polkadot together

---------

Co-authored-by: Mira Ressel <mira@parity.io>

scripts/update-ui-tests.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Script for updating the UI tests for a new rust stable version.
+# Exit on error
+set -e
+
+# by default current rust stable will be used
+RUSTUP_RUN=""
+# check if we have a parameter
+# ./scripts/update-ui-tests.sh 1.70
+if [ ! -z "$1" ]; then
+ echo "RUST_VERSION: $1"
+  # This will run all UI tests with the rust stable 1.70.
+  # The script requires that rustup is installed.
+  RUST_VERSION=$1
+  RUSTUP_RUN="rustup run $RUST_VERSION"
+
+
+  echo "installing rustup $RUST_VERSION"
+  if ! command -v rustup &> /dev/null
+  then
+    echo "rustup needs to be installed"
+    exit
+  fi
+  
+  rustup install $RUST_VERSION
+  rustup component add rust-src --toolchain $RUST_VERSION
+fi
+
+# Ensure we run the ui tests
+export RUN_UI_TESTS=1
+# We don't need any wasm files for ui tests
+export SKIP_WASM_BUILD=1
+# Let trybuild overwrite the .stderr files
+export TRYBUILD=overwrite
+
+# ./substrate
+$RUSTUP_RUN cargo test -p sp-runtime-interface ui
+$RUSTUP_RUN cargo test -p sp-api-test ui
+$RUSTUP_RUN cargo test -p frame-election-provider-solution-type ui
+$RUSTUP_RUN cargo test -p frame-support-test ui