Remove refs to LEDE and use "OpenWrt" instead of "OpenWRT"

Remove instructions on self-checking pull request content for the
following considerations

 - The checks are now enforced by travis autocheck scripts
 - Github now prompts users to refer to the contributing guide on
   submitting new issue and pull request

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

commit message is copied at the end of the template when creating PR,
so swap instructions and template part

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

I hope this will help triage PR and reduce commits not following guidelines

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>