Add notes clarifying what to check out (#122)

Add notes clarifying what to check out
Addresses gh-38.

README.rst

@@ -364,8 +364,8 @@ Multibuild development
 The main multibuild repository is always at
 https://github.com/matthew-brett/multibuild
 
-We try to keep the ``master`` branch stable, so consider doing regular or
-per-build updates of your Multibuild code to current ``master``.
+We try to keep the ``master`` branch stable and do testing and development
+in the ``devel`` branch.  From time to time we merge ``devel`` into ``master``.
 
-We do testing and development in the ``devel`` branch.  From time to time we
-merge ``devel`` into master.
+In practice, you can check out the newest commit from ``devel`` that works
+for you, then stay at it until you need newer features.