Upated CHANGELOG for 0.26.0-rc5.

Upated CHANGELOG for 0.26.0-rc5.

Made v1 API in mesos.proto equivalent to non-v1 for 0.26.0.

Copied missing feature updates to v1 from include/mesos/mesos.proto.

This is a revised version of RR 41046 by Bernd Matthiske.

Review: https://reviews.apache.org/r/41060

Updated CHANGELOG for 0.26.0-rc3.

Updated CHANGELOG for 0.26.0-rc2.

Review: https://reviews.apache.org/r/40777

Updated upgrade documentation with changes in NetworkInfo protobuf.

Review: https://reviews.apache.org/r/40238

Fixed compiler warning in values tests.

Fixed compiler warning in values tests.

Added initial draft of networking user-doc.

Review: https://reviews.apache.org/r/38963

Added `image_providers` flags to configuration.md.

Review: https://reviews.apache.org/r/38749