github: include pull request template

This text is used by GitHub to remind important things to
people sending PRs through the GitHub's web interface.
See here for more information
https://github.com/blog/2111-issue-and-pull-request-templates



It links to the wiki page about submission rules.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>

.github/pull_request_template

+Thanks for your contribution to the LEDE project!
+
+To help keep the codebase consistent and readable,
+and to help people review your contribution,
+we ask you to follow the rules you find in the wiki at this link
+https://lede-project.org/submitting-patches
+
+Please remove this message before posting the pull request.