add explanation of drafts category

contracts/drafts/README.adoc

@@ -1,5 +1,7 @@
 = Drafts
 
+Contracts in this category should be considered unstable. They are as thoroughly reviewed as everything else in OpenZeppelin Contracts, but we have doubts about their API so we don't commit to backwards compatibility. This means these contracts can receive breaking changes in a minor version, so you should pay special attention to the changelog when upgrading. For anything that is outside of this category you can read more about xref:ROOT:api-stability.adoc[API Stability].
+
 NOTE: This page is incomplete. We're working to improve it for the next release. Stay tuned!
 
 == ERC 20

docs/modules/ROOT/pages/api-stability.adoc

@@ -2,7 +2,7 @@
 
 On the https://github.com/OpenZeppelin/openzeppelin-contracts/releases/tag/v2.0.0[OpenZeppelin 2.0 release], we committed ourselves to keeping a stable API. We aim to more precisely define what we understand by _stable_ and _API_ here, so users of the library can understand these guarantees and be confident their project won't break unexpectedly.
 
-In a nutshell, the API being stable means _if your project is working today, it will continue to do so_. New contracts and features will be added in minor releases, but only in a backwards compatible way.
+In a nutshell, the API being stable means _if your project is working today, it will continue to do so_. New contracts and features will be added in minor releases, but only in a backwards compatible way. The exception to this rule are contracts in the xref:api:drafts.adoc[Drafts] category, which should be considered unstable.
 
 [[versioning-scheme]]
 == Versioning scheme