Add information about compat annotations to CONTRIBUTING.md.

CONTRIBUTING.md

@@ -184,6 +184,21 @@ A doctest needs to match an interactive REPL including the `julia>` prompt. To r
 
 For new functionality and other substantial changes, add a brief summary to `NEWS.md`. The news item should cross reference the pull request (PR) parenthetically, in the form `([#pr])`; after adding this, run `./julia doc/NEWS-update.jl` from the `julia` directory to update the cross-reference links. To add the PR reference number, first create the PR, then push an additional commit updating `NEWS.md` with the PR reference number.
 
+#### Annotations for new features, deprecations and behavior changes
+
+API additions and deprecations, and minor behavior changes are allowed in minor version releases.
+For documented features that are part of the public API, a compatibility note should be added into
+the manual or the docstring. It should state the Julia minor version that made changed the behavior
+and have a brief message describing the change.
+
+At the moment, this should always be done with the following `compat` admonition
+(so that it would be possible to programmatically find the annotations in the future):
+
+  ```
+  !!! compat "Julia 1.X"
+      This method was added in Julia 1.X.
+  ```
+
 ### Contributing to core functionality or base libraries
 
 *By contributing code to Julia, you are agreeing to release it under the [MIT License](https://github.com/JuliaLang/julia/tree/master/LICENSE.md).*