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
file.

Change-Id: I7da127fbf36eb3a7cb68e3a91cc9dfbb7fc92155
Reviewed-on: https://boringssl-review.googlesource.com/7370
Reviewed-by: Adam Langley <agl@google.com>