convert: add initial docs for git sources

Upcoming patches will add config options for git sources. This patch adds a
place to document them.

hgext/convert/__init__.py

@@ -282,6 +282,15 b' def convert(ui, src, dest=None, revmapfi'
     :convert.svn.startrev: specify start Subversion revision number.
         The default is 0.
 
+    Git Source
+    ##########
+
+    The Git importer converts commits from all reachable branches (refs
+    in refs/heads) and remotes (refs in refs/remotes) to Mercurial.
+    Branches are converted to bookmarks with the same name, with the
+    leading 'refs/heads' stripped. Git submodules are converted to Git
+    subrepos in Mercurial.
+
     Perforce Source
     ###############
 

tests/test-convert.t

@@ -236,6 +236,14 b''
       convert.svn.startrev
                     specify start Subversion revision number. The default is 0.
   
+      Git Source
+      ##########
+  
+      The Git importer converts commits from all reachable branches (refs in
+      refs/heads) and remotes (refs in refs/remotes) to Mercurial. Branches are
+      converted to bookmarks with the same name, with the leading 'refs/heads'
+      stripped. Git submodules are converted to Git subrepos in Mercurial.
+  
       Perforce Source
       ###############