Add supported compilers section to README.md

It's clang or clang-cl.

Bug: 389951208
Change-Id: I5d2982b33f223e7f756bd40513c9c38c86da6f88
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/128050
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

README.md

diff --git a/README.md b/README.md
index 1aeb563..f6fe623 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,10 @@
 not function properly on big-endian architectures. Bugs and/or patches are
 welcome, however providing this support is **not** a priority at this time.
 
+### Compilers supported
+
+Clang. Former MSVC users should consider using clang-cl if needed.
+
 #### Google employees
 
 Run: `download_from_google_storage --config` and follow the