Added .git-blame-ignore-revs.

This file will be used to record revisions to be ignored by default by
git hyper-blame. For now, it just contains comments outlining some
guidelines on how to append new revisions to the file. It has no actual
revisions.

BUG=574290

Review URL: https://codereview.chromium.org/1705503004

Cr-Commit-Position: refs/heads/master@{#376868}

.git-blame-ignore-revs

@@ -0,0 +1,17 @@
+# git hyper-blame master ignore list.
+#
+# This file contains a list of git hashes of revisions to be ignored by git
+# hyper-blame (in depot_tools). These revisions are considered "unimportant" in
+# that they are unlikely to be what you are interested in when blaming.
+#
+# Instructions:
+# - Only large (generally automated) reformatting or renaming CLs should be
+#   added to this list. Do not put things here just because you feel they are
+#   trivial or unimportant. If in doubt, do not put it on this list.
+# - Precede each revision with a comment containing the first line of its log.
+# - Only put full 40-character hashes on this list (not short hashes or any
+#   other revision reference).
+# - Append to the bottom of the file (revisions should be in chronological order
+#   from oldest to newest).
+# - Because you must use a hash, you need to append to this list in a follow-up
+#   CL to the actual reformatting CL that you are trying to ignore.