Document supported platforms.

Closes issue 183.

Readme.md

@@ -24,6 +24,18 @@ All versions of the extension contain experimental features in the `toolkit_expe
 
 ## 💿 Installing From Source ##
 
+### Supported platforms
+
+The engineering team regularly tests the extension on the following platforms:
+
+- x86_64-unknown-linux-gnu (Ubuntu Linux 20.04) (tested prior to every merge)
+- x86_64-apple-darwin (MacOS 12) (tested frequently on eng workstation)
+- aarch64-apple-darwin (MacOS 12) (tested frequently on eng workstation)
+
+aarch64-unknown-linux-gnu support is [in development](https://github.com/timescale/timescaledb-toolkit/issues/382).
+
+As for other platforms:  patches welcome!
+
 ### 🔧 Tools Setup ###
 
 Building the extension requires valid [rust](https://www.rust-lang.org/), [rustfmt](https://github.com/rust-lang/rustfmt), and clang installs, along with the postgres headers for whichever version of postgres you are running, and pgx.