Describe Volta's SemVer commitments (COMPATIBILITY.md)

Co-authored-by: Rob Jackson <me@rwjblue.com>

Author: chriskrycho

COMPATIBILITY.md

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+# Compatibility
+
+Volta currently tests against the following platforms, and will treat it as a breaking change to drop support for them:
+
+- macOS
+    - x86-64
+    - Apple Silicon
+- Linux x86-64
+- Windows x86-64
+
+We compile release artifacts compatible with the following, and likewise will treat it as a breaking change to drop support for them:
+
+- macOS v11
+- RHEL and CentOS v6
+- Windows 10
+
+In general, Volta should build and run against any other modern hardware and operating system supported by stable Rust, and we will make a best effort not to break them. However, we do *not* include them in our SemVer guarantees or test against them.