[windows] .gitattributes for proper line endings settings (#667)

Context: https://help.github.com/articles/dealing-with-line-endings/

If you work on this repo on Windows, you quickly see the `.gitattributes` are not setup right for development on Windows.

1. Certain files had mixed line endings, which causes Visual Studio to prompt to adjust.

2. Running `premake` yields Windows line endings on Windows, and so every `*.csproj` file appears to have changes.

`xamarin-android` has a well-used `.gitattributes` file, so I used it as a starting point and added an entry for `*.cake` files.

The way it works (as I understand it):
- `git` stores everything with unix line endings
- on checkout, clients convert the line endings according to settings in the `.gitattributes` file

So, for example:
- All `*.csproj` files will have Windows line endings, as Visual Studio expects (both Mac and Windows)
- `*.cs` files will have Windows line endings on Windows, and unix line endings on Mac

.gitattributes

@@ -0,0 +1,33 @@
+# This file mainly controls line ending conversion behaviour, if
+# the user has the setting core.autocrlf true.
+
+# The meaning of the attributes is a little odd
+# -crlf means DO NOT convert line endings
+# crlf means CONVERT to lf in the repo, but always checkout using crlf
+# on all platforms
+
+# sln is always CRLF, even on linux, so don't convert
+*.sln	eol=crlf
+*.bat	eol=crlf
+*.cmd	eol=crlf
+*.rtf	eol=crlf
+
+# Mostly generated by VS, so avoid extra noise
+*.Designer.cs	eol=crlf
+
+*.cs	text
+*.cake	text
+*.xml	text
+*.md	text
+Makefile	eol=lf
+*.targets	eol=crlf
+*.proj	eol=crlf
+*.vcproj	eol=crlf
+*.vcxproj	eol=crlf
+*.csproj	eol=crlf
+*.shproj	eol=crlf
+*.projitems	eol=crlf
+*.tpnitems	eol=crlf
+*.wixproj	eol=crlf
+*.wxs	eol=crlf
+*.rtf	eol=crlf