README.md:
 * Update "About SystemC" wording from
   https://accellera.org/community/systemc/
 * proof-of-concept -> reference
 * Move issue reporting details to `CONTRIBUTING.md`

CONTRIBUTING.md:
 * Remove internal development details
   (re-added to DEVELOPMENT.md)
 * Add issue reporting section
 * Keep repo organization section (slightly shortened)

docs/DEVELOPMENT.md:
 * Re-add internal documentation file
 * Some cleanups

(cherry picked from commit 2ff01a4ba59b8ef547d4aa8caf0f54e0bad1d2b9)