doc: add mention of indexmap feature (#349)

Co-authored-by: dj8yf0μl <[email protected]>

Author: dj8yfo

borsh/docs/rustdoc_include/borsh_crate_top_level.md

@@ -43,6 +43,9 @@
 * **bson** -
   Gates implementation of [BorshSerialize] and [BorshDeserialize]
   for [ObjectId](https://docs.rs/bson/2.9.0/bson/oid/struct.ObjectId.html).
+* **indexmap** -
+  Gates implementation of [BorshSerialize] and [BorshDeserialize]
+  for [indexmap::IndexMap](https://docs.rs/indexmap/2.8.0/indexmap/map/struct.IndexMap.html) and [IndexSet](https://docs.rs/indexmap/2.8.0/indexmap/set/struct.IndexSet.html)
 * **ascii** -
   Gates implementation of [BorshSerialize], [BorshDeserialize], [BorshSchema] for
   types from [ascii](https://docs.rs/ascii/1.1.0/ascii/) crate.