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
Fix typos in XML comments
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
System.Xml.XmlDocument.Tests formatter changes
System.Xml.XmlDocument.Tests Sanitizer changes
Replacing foo:bar with val1:val2
Restoring TODOs in comments
Merge branch 'xmltypo'
Remove unused using statements
Update Developers.md

Fixing the name of and adding a link to Visual Studio Community 2013.
Merge remote-tracking branch 'upstream/master' for SetNamedItemTests
Sort usings.
Merge branch 'remove-unused-usings'
Fix build break due to using being removed that was actually used.
Documentation comments: CDATA to {T} for generics

Replaced CDATA with {T} syntax for generics in documentation comments.