Add examples/README.md to explain and provide steps to create missing project/make files

An issue that seems to trip up users new and old seems to be the missing example project files. I cannot count the number of times I've tried to help a user out on IRC or the forum that has run into this and I really feel as though we are making things more difficult for end-users. The main README.md does reference the issue:

If you are working with the Git repository, the stable branch of the OF repository corresponds to the most recent release, with a few important differences:
…
3. This GitHub repository has no project files for the different examples. They are generated automatically for each release using a tool in apps/devApps/projectGenerator/.
If you want to work with the openFrameworks GitHub repository, you should use the project generator to create project files for all the code in examples/.

but this isn't particularly helpful as it is hiding behind a "developers" badging (What if they are just a user and not a developer? Does it mean developers of openFrameworks proper or developers of applications using openFrameworks?), this probably isn't descriptive enough for new users and this doesn't help users that downloaded a repo archive via GitHub.

I suspect there are good reasons for not having the project/make files in the repo itself (though if it is file size, one could argue a couple of MB of project files pales in comparison to the trillion MBs of third party dependencies 😉) but I think it would be kind to pull out the issue to top billing in the main README and create a secondary README right in the examples folder as well. I am a bit behind on the current state of PG, command-line vs GUI, submodule vs inline, so I'm not in a great position to offer the README tweaks, but I do think it is an issue I'd hope could be considered for 0.9.0 if others felt important.

Note: the examples/README.md would probably need to be stripped for the nightly builds.

Start with the Developers section of README.md and the project generator under apps/devApps/projectGenerator/ to confirm the current command-line, GUI, and repository workflow. Add examples/README.md and improve the main README guidance so users can create missing project or make files, while checking how nightly builds should handle the added file.