Updated README.md.
Updated README.md.

Specified GCC release information (written post-release) and (upcoming) Sourcery support for advanced development of OpenCoarrays.
zbeekman/opencoarrays,sourceryinstitute/opencoarrays,jerryd/opencoarrays,jerryd/opencoarrays,zbeekman/opencoarrays,zbeekman/opencoarrays,jerryd/opencoarrays,sourceryinstitute/opencoarrays,sourceryinstitute/opencoarrays