Add PyCharm project directory into the .gitignore

Change-Id: I9a785a9d045e44c6ec8bd4bd8d0169a81d5ccfde
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/251835
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Rostislav Krasny <rostigm@gmail.com>

.gitignore

@@ -6,3 +6,6 @@ __pycache__
 .repopickle_*
 /repoc
 /.tox
+
+# PyCharm related
+/.idea/