flake8: Add comments in config to explain suppressed checks

Change-Id: Ib5c09b36d40a96ba9167b42b3bd2f1ed072660b7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254611
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>

.flake8

@@ -1,3 +1,13 @@
 [flake8]
 max-line-length=100
-ignore=E111,E114,E402,W503,W504
+ignore=
+    # E111: Indentation is not a multiple of four
+    E111,
+    # E114: Indentation is not a multiple of four (comment)
+    E114,
+    # E402: Module level import not at top of file
+    E402,
+    # W503: Line break before binary operator
+    W503,
+    # W504: Line break after binary operator
+    W504