Rollup merge of rust-lang#64621 - XAMPPRocky:relnotes, r=Mark-Simulacrum

Add Compatibility Notes to RELEASES.md for 1.38.0

### [Rendered](https://github.com/XAMPPRocky/rust/blob/relnotes/RELEASES.md)

r? @Mark-Simulacrum

Author: tmandry

RELEASES.md

@@ -68,6 +68,13 @@ Misc
 - [`rustc` will now warn about some incorrect uses of
   `mem::{uninitialized, zeroed}` that are known to cause undefined behaviour.][63346]
 
+Compatibility Notes
+-------------------
+- Unfortunately the [`x86_64-unknown-uefi` platform can not be built][62785]
+  with rustc 1.39.0.
+- The [`armv7-unknown-linux-gnueabihf` platform is also known to have
+  issues][62896] for certain crates such as libc.
+
 [60260]: https://github.com/rust-lang/rust/pull/60260/
 [61457]: https://github.com/rust-lang/rust/pull/61457/
 [61491]: https://github.com/rust-lang/rust/pull/61491/
@@ -79,7 +86,9 @@ Misc
 [62735]: https://github.com/rust-lang/rust/pull/62735/
 [62766]: https://github.com/rust-lang/rust/pull/62766/
 [62784]: https://github.com/rust-lang/rust/pull/62784/
+[62785]: https://github.com/rust-lang/rust/issues/62785/
 [62814]: https://github.com/rust-lang/rust/pull/62814/
+[62896]: https://github.com/rust-lang/rust/issues/62896/
 [63000]: https://github.com/rust-lang/rust/pull/63000/
 [63056]: https://github.com/rust-lang/rust/pull/63056/
 [63107]: https://github.com/rust-lang/rust/pull/63107/