Update README.md

LInk to the official page of the project.
Merge branch 'release'
Configure the project assembly signing.
Update output directory of signed builds.
Fix NuSpec configuration on signed builds.
Fix xml documentation file name on signed builds.
Only restore packages needed to run unit tests.
Add missing parameter to nuget restore.
Remove samples project because samples are now available on DotNetFiddle.
Remove old support tools.
Add portable class library target.
Update build configuration.
Fix build errors.
Fix nuget package version numbers.
Update changelog.