Add git implementation flag and note about Azure DevOps

Signed-off-by: Philip Laine <philip.laine@gmail.com>

docs/cmd/flux_create_source_git.md

@@ -56,6 +56,7 @@ flux create source git [name] [flags]
 
 ```
       --branch string                          git branch (default "master")
+      --git-implementation string              the git implementation to use, can be 'go-git' or 'libgit2'
   -h, --help                                   help for git
   -p, --password string                        basic authentication password
       --secret-ref string                      the name of an existing secret containing SSH or basic credentials