Rephrase the template a bit and add information about
older releases and forks.

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>

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>

So that maintainers can receive at least one notification at the issue
creation time ;)

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