Add FullGroupJoin to release notes

2.5 version stamp

Note about maintenance release

Pin projects to C# version 7

Add Partition; closes morelinq#210

Partition splits a sequence in two or more generally groupings

.NET 4.0 can be targeted with System.ValueTuple

FillBackward overload missing return doc; closes morelinq#268

Add SkipLast; closes morelinq#215

Adds Sequence(start, stop, [step]); closes morelinq#240

Swap FillForward/Backward filler function args

It is more natural to supply the "current" item first.