Add compat notes.

compat.md

@@ -0,0 +1,19 @@
+# WebAuthn Compat Notes
+
+A document to track more specific nuances than <https://caniuse.com/?search=webauthn>
+## macOS
+
+| Functionality | Links | Chrome | Firefox | Safari |
+|-|-|-|-|-|
+| `PublicKeyCredential.isConditionalMediationAvailable()` | [spec](https://w3c.github.io/webauthn/#dom-publickeycredential-isconditionalmediationavailable), [crbug](https://bugs.chromium.org/p/chromium/issues/detail?id=1330946) | 🏁 106.0.5220.0+[^1] | ❌ | ✅ 16+ |
+
+[^1]: Flag: `chrome://flags/#webauthn-conditional-ui`
+
+- ✅ Fully implemented and available *without* a custom setting/flag.
+- 🏁 Fully implemented and available *only with* a custom setting/flag.
+- 🚧 Partially implemented or implementation drafted (may or may not be behind a flag).
+- ❌ Unimplemented
+
+## Useful links
+
+- Look up what Chrome versions a commit is in: <https://omahaproxy.appspot.com/> ("Find Releases")