glossary: explain "master" and "origin"

If you are a long time git user/developer, you forget that to a new git
user, these words have not the same meaning as to you.

[jc: with updates from J. Bruce Fields.]
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/glossary.txt

@@ -111,6 +111,17 @@ branch::
 	a particular revision, which is called the branch head. The
 	branch heads are stored in `$GIT_DIR/refs/heads/`.
 
+master::
+	The default branch. Whenever you create a git repository, a branch
+	named "master" is created, and becomes the active branch. In most
+	cases, this contains the local development.
+
+origin::
+	The default upstream branch. Most projects have one upstream
+	project which they track, and by default 'origin' is used for
+	that purpose.  New updates from upstream will be fetched into
+	this branch; you should never commit to it yourself.
+
 ref::
 	A 40-byte hex representation of a SHA1 pointing to a particular
 	object. These may be stored in `$GIT_DIR/refs/`.