Updated change log for 3.6.1 release

Protocol Buffers v3.6.0.1

Protocol Buffers v3.6.0

Merge pull request protocolbuffers#4702 from TeBoring/3.6.x

Cherry-pick changes into 3.6.x

Merge pull request protocolbuffers#4673 from acozzette/memory-leak-fix

Make sure to delete temporary maps used by FileDescriptorTables

Bump version number to 3.5.2

Update version number to 3.5.1.1

Merge pull request protocolbuffers#4073 from pherl/changelog

Update changelog for 3.5.1

Add release log

Merge pull request protocolbuffers#3868 from pherl/3.5.x

Support win32 long path for cross compiled build