Update readme to mention unstable features and nightly MSRV

Author: nicholasbishop

README.md

@@ -97,6 +97,11 @@ prerequisites for running the tests.
 
 For instructions on how to create your own UEFI apps, see the [BUILDING.md](BUILDING.md) file.
 
+The uefi-rs crates currently require some [unstable features].
+The nightly MSRV is currently 2022-04-18.
+
+[unstable features]: https://github.com/rust-osdev/uefi-rs/issues/452
+
 ## Contributing
 
 We welcome issues and pull requests! For instructions on how to set up a development