Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
tests(System.Numerics.Vectors/tests): vectors tests should work non non en-US culture environment
Fix typo in InitializeNestedTypesMap
Added "readonly" when possible
Removing redundant comparison with "null" ((!(obj is Vector)) is superset condition for (obj == null))
Reduce number of TLS accesses in AllocFreeConcurrentStack<T>
Fix typos in XML comments
check for null parameter as base class does, but throw ArgumentException
Merge branch 'fix-vectors-tests' of https://github.com/jbuiss0n/corefx into fix-vectors-tests
Merge branch 'master' of https://github.com/AndreyAkinshin/corefx into remove-null-check
Merge branch 'issue-43' of https://github.com/andi9310/corefx into add-readonly
Merging content between contributing.md and developers.md
Updated .gitignore

 - Visual Studio temporary files, build- and test results
 - Files generated by popular Visual Studio extensions
 - Windows generated- and system files
 - Linux generated- and system files
 - OSX generated- and system files
Merge branch 'typo-md-reader' of https://github.com/jbevain/corefx
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'