Add merge-base to compatibility table

Signed-off-by: David Pordomingo <David.Pordomingo.F@gmail.com>

Author: dpordomingo

COMPATIBILITY.md

@@ -86,7 +86,7 @@ is supported by go-git.
 | for-each-ref                          | ✔ |
 | hash-object                           | ✔ |
 | ls-files                              | ✔ |
-| merge-base                            | |
+| merge-base                            | ✔ | Calculates the merge-base only between two commits, and supports `--independent` and `--is-ancestor` modifiers; Does not support `--fork-point` nor `--octopus` modifiers. |
 | read-tree                             | |
 | rev-list                              | ✔ |
 | rev-parse                             | |