Update README.md

Co-authored-by: Chris Holmes <[email protected]>

Author: Phil Varner

README.md

@@ -28,7 +28,9 @@
 The [main](https://github.com/radiantearth/stac-api-spec/tree/main) branch in GitHub is
 used for active development and may be unstable. Implementers should reference one of
 the release branches above for a stable version of the specification.
-
+*NOTE: This means that if you are on github.com/radiantearth/stac-api-spec then you are looking at an unreleased,
+unstable version of the specification. Use the first listed link on releases to read the current released, stable version
+of the spec.
 ## About
 
 The SpatioTemporal Asset Catalog (STAC) family of specifications aim to standardize the way geospatial asset metadata is structured and queried.