README.md: link to external formatting of user manual

Some people don't like htmlpreview.github.io.  I once or twice saw a
case where it appeared to be affected by load limits.  Since external
sites are making the manual available, and it's unlikely there are too
many changes between the last release and the current manual, just link
to it as an alternative for folks.

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

Documentation/git-filter-repo.txt

@@ -1,7 +1,9 @@
 // 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.
+// to access the actual documentation, or view another site that
+// has compiled versions available, such as:
+//     https://www.mankier.com/1/git-filter-repo
 
 git-filter-repo(1)
 ==================

README.md

@@ -73,6 +73,11 @@ section](https://htmlpreview.github.io/?https://github.com/newren/git-filter-rep
     commands](Documentation/converting-from-bfg-repo-cleaner.md#cheat-sheet-conversion-of-examples-from-bfg),
     which covers every example from the BFG website
 
+External sites also have [alternative formats of the user
+manual](https://www.mankier.com/1/git-filter-repo) available, at least
+for the most recent release.  This also may be beneficial if
+htmlpreview.github.io starts hitting load limits.
+
 # Why filter-repo instead of other alternatives?
 
 This was covered in more detail in a [Git Rev News article on