Update CONTRIBUTING to better reflect current branch strategy

This commit clarifies that both new features and bug fixes should be
submitted to the "develop" branch initially, with changes moving
from "develop" to "master" only when it is time for a new release.
It also clarifies that pull requests should always be done against
the "develop" branch.

CONTRIBUTING.rst

@@ -68,19 +68,18 @@ contributors list.
 Branches
 --------
 
-There are two long-lived branches in AsyncSSH at the moment:
+There are two long-lived branches in AsyncSSH:
 
 * The master branch is intended to contain the latest stable version
   of the code. All official versions of AsyncSSH are released from
   this branch, and each release has a corresponding tag added
-  matching its release number. Bug fixes and simple improvements
-  may be checked directly into this branch, but most new features
-  will be added to the develop branch first.
-
-* The develop branch is intended to contain features for developers
-  to test before they are ready to be added to an official release.
-  APIs in the develop branch may be subject to change until they
-  are migrated back to master, and there's no guarantee of backward
+  matching its release number.
+
+* The develop branch is intended to contain new features and bug fixes
+  ready to be tested before being added to an official release. APIs
+  in the develop branch may be subject to change until they are
+  migrated back to master, and there's no guarantee of backward
   compatibility in this branch. However, pulling from this branch
   will provide early access to new functionality and a chance to
-  influence this functionality before it is released.
+  influence this functionality before it is released. Also, all
+  pull requests should be submitted against this branch.