update README to indicate the changes that were made to LMDB in this repository

Author: matthewbastien

README.md

@@ -6,6 +6,12 @@ This wrapper provides exposure of the library's C interfaces to Swift, allowing
 
 Changes in LMDB will be pulled into this repository as the upstream project evolves. The LMDB repository is mirrored here: <https://github.com/LMDB/lmdb>.
 
+## Changes to LMDB
+
+Swift LMDB makes two local changes to LMDB in order to pass thread sanitization checks in [IndexStoreDB](https://github.com/swiftlang/indexstore-db). These changes are:
+ - Use `_Atomic` instead of `volatile` for transaction types
+ - Build with POSIX mutexes enabled and robust mutexes disabled on Linux and macOS
+
 ## License
 
 The included LMDB database project is released under [The OpenLDAP Public License](https://git.openldap.org/openldap/openldap/-/blob/mdb.master/libraries/liblmdb/LICENSE). This same license applies to the Swift code included in this repository.