Update README with how to install ILRepack as a dotnet global tool

[fourpastmidnight]

I recently saw that as part of the 2.0.35 release, it's supposed to be possible to install ILRepack as a dotnet global tool.

So I issued the command dotnet tool install -g ilrepack but was met with the following error:

Tool 'ilrepack' failed to update due to the following:
The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package.
Tool 'ilrepack' failed to install. Contact the tool author for assistance.

[fourpastmidnight]

OK, so because dotnet tools share Nuget.org as their repository, of course ilrepack didn't install as a dotnet tool, because it's not a dotnet tool but a regular old Nuget package.

The dotnet tool needs to be installed with dotnet tool install -g dotnet-ilrepack. It would be nice if the README mentioned this 😄