FullGroupJoin TSecond type param doc typo; fixes morelinq#349

List From in README & package description

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