Added crate-level docs for the parachains pallet (#1772)

* crate-level docs for the parachains pallet

* fix typos

bridges/modules/grandpa/README.md

@@ -29,6 +29,7 @@ There are two main things in GRANDPA that help building light clients:
 - there's no need to import all headers of the bridged chain. Light client may import finalized headers or just
   some of finalized headders that it consider useful. While the validators set stays the same, the client may
   import any header that is finalized by this set;
+
 - when validators set changes, the GRANDPA gadget adds next set to the header. So light client doesn't need to
   verify storage proofs when this happens - it only needs to look at the header and see if it changes the set.
   Once set is changed, all following justifications are generated by the new set. Header that is changing the
@@ -69,14 +70,31 @@ There may be a special account in every runtime where the bridge GRANDPA module
 account, named 'module owner', is like a module-level sudo account - he's able to halt and
 resume all module operations without requiring runtime upgrade. Calls that are related to this
 account are:
+
 - `fn set_owner()`: current module owner may call it to transfer "ownership" to another account;
+
 - `fn set_operating_mode()`: the module owner (or sudo account) may call this function to stop all
   module operations. After this call, all finality proofs will be rejected until further `set_operating_mode` call'.
   This call may be used when something extraordinary happens with the bridge;
+
 - `fn initialize()`: module owner may call this function to initialize the bridge.
 
 If pallet owner is not defined, the governance may be used to make those calls.
 
+## Signed Extension to Reject Obsolete Headers
+
+It'd be better for anyone (for chain and for submitters) to reject all transactions that are submitting
+already known headers to the pallet. This way, we leave block space to other useful transactions and
+we don't charge concurrent submitters for their honest actions.
+
+To deal with that, we have a [signed extension](./src/extension.rs) that may be added to the runtime.
+It does exactly what is required - rejects all transactions with already known headers. The submitter
+pays nothing for such transactions - they're simply removed from the transaction pool, when the block
+is built.
+
+You may also take a look at the [`generate_bridge_reject_obsolete_headers_and_messages`](../../bin/runtime-common/src/lib.rs)
+macro that bundles several similar signed extensions in a single one.
+
 ## GRANDPA Finality Relay
 
 We have an offchain actor, who is watching for GRANDPA justifications and submits them to the bridged chain.