[Documentation] Add usage example of msbuild-architecture input

[LanceMcCarthy]

Add description and examples of using the msbuild-architecture input.

For example:

    - name: Setup 64 bit msbuild.exe
      uses: microsoft/setup-msbuild@v1.1
      with:
        msbuild-architecture: 'x64'

and

    - name: Setup 32 bit msbuild.exe
      uses: microsoft/setup-msbuild@v1.1
      with:
        msbuild-architecture: 'x86'

It would also be nice to see the other parameter descriptions/examples. If you're updating it for this one, might as well add any missing ones while you're in there :)

[MarkIngramUK]

vs-prerelease isn't mentioned in the readme either. I had to dig through the source code to find out if it was possible.