[FIX] CONTRIBUTING.md: fix and improve the contribution docu

- one place still said we are not accepting pull requests

Change-Id: I1d547cbfe66616767057528d45725b235ef7d603
[INTERNAL] sap.ui.ux3.Shell: Fix QUnit test

Change-Id: I26546fb166a3f5fdf43a5ff2c9444b3bfaf1cd14
Merge "[FIX] CONTRIBUTING.md: fix and improve the contribution docu"