doc: update release process with branching

Add extra step with branch creation after stable releases, to be used
for doing patch releases.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
diff --git a/docs/release.md b/docs/release.md
index 3115b6d..ff00cec 100644
--- a/docs/release.md
+++ b/docs/release.md
@@ -41,10 +41,11 @@
 requires that its version to be updated by editing
 `scripts/imgtool/__init__.py` and modifying the exported version:
 
-`imgtool_version = "X.Y.ZrcN"`
+`imgtool_version = "A.B.CrcN"`
 
-where `rcX`, `aX` and `bX` are accepted pre-release versions (just
-numbers for final releases). For more info see:
+This version should match the current release number of MCUboot; `rcN`
+(with no dash!) is accepted for pre-release version under test, and
+numbers only for final releases. For more info see:
 
 https://www.python.org/dev/peps/pep-0440/#pre-releases
 
@@ -84,6 +85,20 @@
 git push origin va.b.c-rcn
 ```
 
+## Branching after a Release
+
+After the final (non-`rc`) a.b.0 release is made, a new branch must
+be created and pushed:
+
+``` bash
+git checkout va.b.c
+git checkout -b va.b-branch
+git push origin va.b-branch
+```
+
+This branch will be used to generate new incremental `PATCH` releases
+for bug fixes or required minor updates (eg, new `imgtool` features).
+
 ## Post release actions
 
 Mark the MCUBoot version as a development version. The version number used