Clarify we don't use any Git service issue tracking
Clarify we don't use any Git service issue tracking

Reword the "no Git issue tracking" policy in contributing section.

Previously the contributing section in README.md would say we don't use
git.pantsu.cat's issue tracker to track issues. That Git service
repository's issues have been disabled today, so we might as well say we
don't use any Git service issue tracking. Unofficial mirrors like the
one on notabug.org may still have issue tracking, which I believe we
have no control over.

This commit also clarifies that we do it for ethical and technical
objections. (In short, it's because the Git service is centralized and a
walled garden.) For more information, see
wubthecaptain/deathwatch-pomf@fc86cbf for information why.

alucard0134/Pomf,alucard0134/Pomf,pomf/pomf-php,pomf/pomf,alucard0134/Pomf,ewhal/gpomf,pomf/pomf