UserManual: Rewrite most parts for GitX 0.7

This rewrites most of the UserManual for GitX 0.7 and also updates the
screenshots to reflect the latest GitX-version.

Signed-off-by: Johannes Gilger <[email protected]>

Author: heipei

Site/UserManual/assets/images/branch-lanes.png

@@ -1,8 +1,9 @@
 User Manual
 -----------
 
-This is the GitX user's manual. In this document, you will be able to read
-about everything you need to know on how to use GitX.
+This is the GitX user's manual. We try to document the most-commonly used 
+features here and explain concepts that might be confusing if you've never used 
+a git-gui.
 
 ### Disclaimer
 
@@ -13,4 +14,6 @@ git command-line programs with their ever-growing number of options. It's also
 no easy way to learn git. A sound understanding of how git stores data, how it 
 works on that data and how to recover from (accidental) deletion of branches 
 and commits will not only help you with git but will also enable you to learn 
-GitX with greater confidence and speed.
+GitX with greater confidence and speed. Since GitX is far from complete you'll 
+probably still have to use it in conjunction with the command-line git for more 
+complex tasks.