Documentation and dependencies for the v42r6p1 release

ReleaseNotes/v42r6p1.md

+2017-09-12 DaVinci v42r6p1
+==========================
+
+Release for 2017 production
+---------------------------
+
+This version is released on the 2017-patches branch (but master as far as DaVinci goes).
+It is based on Gaudi v28r2, LHCb v42r6p1, Rec v21r6p1, Phys v23r6p1, Analysis v18r5p1 and Stripping v11r3p1.
+Uses LCG_88 with ROOT 6.08.06.
+
+- Many tests updated and/or fixed.
+  - See merge requests !95, !93