For #15273: add warnings about codeowners; add perf team as CODEOWNERS owner.

.github/CODEOWNERS

@@ -14,6 +14,12 @@
 # to their GitHub account, for example user@example.com.
 # https://help.github.com/articles/about-codeowners/
 
+# WARNING: if there is a single syntax error in this file, CODEOWNERS
+# WILL NOT WORK AT ALL. Please be careful when editing this file.
+#
+# You can use the technique described in this blog post to validate
+# the paths you specify in .gitignore:
+# http://www.benjaminoakes.com/git/2018/08/10/Testing-changes-to-GitHub-CODEOWNERS/
 
 # By default the Android Components team will be the owner for everything in
 # the repo. Unless a later match takes precedence.
@@ -30,6 +36,12 @@
 # these changes (for now) but to be aware of them. Please let us know
 # if the CODEOWNERS system makes this impractical. We're available at
 # #perf-android-frontend on Matrix.
+
+# The perf team is relying on CODEOWNERS to catch regressions. If
+# there is a single syntax error in the file, no rules will work.
+# Therefore, we make the Perfomance team code owners of this file.
+/.github/CODEOWNERS @mozilla-mobile/Performance
+
 /app/src/*/java/org/mozilla/fenix/perf/** @mozilla-mobile/Performance
 *.pro @mozilla-mobile/Performance
 *proguard* @mozilla-mobile/Performance