Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
Add an .editorconfig file

Just a start. Could help enforcing the Coding Style
https://github.com/Microsoft/dotnet/blob/master/CONTRIBUTING.md#c-coding-style

What is .editorconfig:
http://editorconfig.org/#overview

Plugins for IDEs to support:
http://editorconfig.org/#download
Added trim_trailing_whitespace = true
Add optimization for ImmutableList<T>.AddRange when adding non-empty range
to non-empty list
check for null parameter as base class does, but throw ArgumentException
Apply same optimization for ImmutableList<T>.InsertRange, also add more
  test coverage
XmlAttributeCollection.SetNamedItem(XmlNode) returns null if provided a null parameter
Update Developers.md

Fixing the name of and adding a link to Visual Studio Community 2013.
Merge remote-tracking branch 'upstream/master' for SetNamedItemTests
Documentation comments: CDATA to {T} for generics

Replaced CDATA with {T} syntax for generics in documentation comments.
Make InsertRange rebalance list correctly
Change Node.Root to internal to facilitate testing
Revert changes to InsertRangeTest
Add more tests
Fixing System.Xml.XPath.XDocument implementation
Update Developers.md

Visual Studio Community and Visual Studio 2015 Preview text and links
Merge branch 'xml-check-not-null' of https://github.com/jltrem/corefx into jltrem-xml-check-not-null
Update Developers.md

Add text and links for Visual Studio Community and Visual Studio 2015 Preview.
Merge branch 'patch-1' of https://github.com/jac009/corefx
Add doc comment for BalanceNode method
Add license header to test source files.
Add ImmutableArrayExtensions.SelectMany extension method
Optimized ImmutableArrayExtensions.SelectMany method for a zero-length source array.