Minor documentation update for filter and kind variables
Merge pull request #272 from csexton/doc-filter-kind-vars

Minor documentation update for filter and kind variables
- Updated README.
- Fixed grep behavior.
- Updated screenshot.
- Improved FAQ section.
- Improved jump list.
- Improved grep source behavior.
- Added FAQ section.
- Fixed error in file_rec/async.
- Improved sorter_rank behavior(experimental).
- Improved sorter_rank.
- Added FAQ section.
- Display kinds in action list.
- Improved action source.
- Added example.
- Fixed smart case behavior.
- Fixed max_candidates feature.
- Optimized sorter_rank by if_lua.
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