Project Generator submodule / install from GitHub process

The instructions to install from GitHub make a half-hearted attempt at people compile their own PG through the submodule, while acknowledging it's not a good idea:

As the first option can lead to unexpected results, we suggest you use the second approach.

that's kind of noisy and not very elegant...

the PG is compiled in a nightly process, and someone tracking git will be interested in the latest PG anyway, which can be obtained for the main platforms: https://github.com/openframeworks/projectGenerator/releases/tag/nightly and someone wanting to compile PG can go to it as a separate project.

so i propose to remove the instructions to try to compile and simply point to the nightlies PG binaries. this focuses the problems of compiling PG in the Project Generator GitHub README.

Open the Project Generator GitHub README and find the installation instructions that describe compiling through the submodule. Review the linked nightly release for the available platform binaries, then update the guidance so it points users there and directs people who want to compile the Project Generator to its separate project. Done means the README no longer recommends the submodule build path.