Bugfix or docfix: README.md documentation incorrect or script.Assembly property needs to be changed from protected level to public

[AtwoodTM]

In README.md, the example provided in the section "Compiling and Executing Entire Classes" notes to use the following if you need access to the assembly:

var assembly = script.Assembly

This does not work as an CS0122 error is thrown stating "'CSharpScriptExecutionAssembly' is inaccessible due to its protection level."

I would submit a PR to change from protected to public, but was not sure if there was a specific reason for it to be designated as protected.