git-filter-repo.txt: point people at the generated documentation

People keep trying to read this file, unaware that it is the source code
for generating the documentation, not the generated documentation.  Add
a comment at the top that explains this and points people in the right
direction.

Signed-off-by: Elijah Newren <newren@gmail.com>

Documentation/git-filter-repo.txt

@@ -1,3 +1,8 @@
+// This file is NOT the documentation; it's the *source code* for it.
+// Please follow the "user manual" link under
+//     https://github.com/newren/git-filter-repo#how-do-i-use-it
+// to access the actual documentation.
+
 git-filter-repo(1)
 ==================