filter-repo (README): restructure and update

Since we now have a separate user manual and it does not make sense to
duplicate information in multiple places, restructure the README:
  * Refer to the actual manual early on
  * Limit the README to mostly be about why I wrote it and why folks
    might want to consider it instead of existing tools
  * Include a new section on upstream improvements, especially since it
    looks like inclusion of git-filter-repo in git.git is unlikely.

Signed-off-by: Elijah Newren <[email protected]>

Author: newren