Add an introduction about Git branches to the build doc.

Too many new contributors try to build from master.

wiki/src/contribute/build.mdwn

@@ -2,6 +2,13 @@
 
 [[!toc levels=2]]
 
+Git repository and branches
+===========================
+
+You will need to clone the Tails Git repository, and to checkout the
+branch that you want to build (most likely, _not_ `master`): learn
+more about [[our Git branches layout|contribute/git#main-repo]].
+
 <a id="vagrant"></a>
 
 Using Vagrant