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
Apply settings only to *.cs files

Refs https://github.com/dotnet/corefx/pull/56#issuecomment-63719328
Added indendaton rules for *.csproj and *.nuspec

Refs https://github.com/dotnet/corefx/pull/56#issuecomment-63198357
Remove System.Collections.Immutable.xml from under the nuget folder
The documentation in this file is written by technical writers and does not come from the xml comments in the library. The nuspec file uses the xml generated by the library and not this one. Removing the file.
Update the Appveyor Status Badge.

Currently the status badge points to all the builds triggered in the Appveyor including the Pull Request builds. So if a PR build fails the build status on readme.md will show failure. This change will make the status badge in readme.md to point to the latest build on the dotnet/corefx master branch (excluding the PR builds).
Update .gitignore

- MonoDevelop generated files.
Typo fixes in MetadataStringDecoder
Merge branch 'master' of https://github.com/dotnet/corefx
Significant enumeration perf improvements for immutable lists, dictionaries, and sets

These are the approximate improvements I see on my machine when enumerating various
immutable collection types of length 0, 1, and 1000.  The numbers are old
time divided by new time...

ImmutableList:
    Empty - 1.6x
    Single element - 2.2x
    1000 elements - 2.9x

ImmutableHashSet:
    Empty - 1.3x
    Single element - 1.7x
    1000 elements - 1.6x

ImmutableSortedSet:
    Empty - 1.9x
    Single element - 2.5x
    1000 elements - 2.7x

ImmutableDictionary:
    Empty - 1.2x
    Single element - 1.6x
    1000 elements - 1.4x

ImmutableSortedDictionary:
    Empty - 2x
    Single element - 2.5x
    1000 elements - 2.75x
Address CR feedback on previous commit
Merge pull request #135 from stephentoub/immutable_enum_perf

Perf improvements for enumerating immutable collections
Merge pull request #134 from jzebedee/master

Typo fixes in MetadataStringDecoder
Merge pull request #133 from soimort/update-gitignore

Update .gitignore
Merge pull request #132 from Sridhar-MS/appveyor-badge-update

Update the Appveyor Status Badge.
Fix build on 32-bit Windows
Merge pull request #138 from PatrickMcDonald/32-bit-build

Fix build on 32-bit Windows
Fixed some wrong documentation on System.Reflection.
Merge pull request #139 from theCrispy/master

Fixed some wrong documentation on System.Reflection.
Add Rebuild target to build.proj