README.md: document the supported compilers

README.md

@@ -77,11 +77,21 @@ You should compile both `lib/*.c` and `lib/*/*.c`.  You don't need to worry
 about excluding irrelevant architecture-specific code, as this is already
 handled in the source files themselves using `#ifdef`s.
 
-It is strongly recommended to use either gcc or clang, and to use `-O2`.
-
 If you are doing a freestanding build with `-ffreestanding`, you must add
 `-DFREESTANDING` as well (matching what the `CMakeLists.txt` does).
 
+## Supported compilers
+
+Currently the following compilers are supported for building libdeflate:
+
+- gcc: v5.1 or later
+- clang: v3.9 or later
+- MSVC: Visual Studio 2019 or later
+- Other compilers: any version should work
+
+gcc or clang is recommended.  The recommended optimization level is `-O2`.  In
+some cases compilers older than those listed above may work, but are untested.
+
 # API
 
 libdeflate has a simple API that is not zlib-compatible.  You can create