Add a PULL_REQUEST_TEMPLATE.
Now that we have a GitHub mirror, set up a PULL_REQUEST_TEMPLATE so people know
not to file pull requests against us. Text borrowed from Go's version of this
Reviewed-by: Adam Langley <email@example.com>
diff --git a/.github/PULL_REQUEST_TEMPLATE b/.github/PULL_REQUEST_TEMPLATE
new file mode 100644
@@ -0,0 +1,7 @@
+Please do not send pull requests to the BoringSSL repository.
+We do, however, take contributions gladly.