Reference to ./build_re2.sh

[eberdahl]

README.md refers to
./build_re2.sh
which isn't needed anymore.

[shimpe]

More in general, please update the build instructions as currently it's rather difficult getting started.

[mxa]

The build instructions a outdated, Introjucer is not existing any more.

[mxa]

We have simplified the installation, brought the submodules up to date and forked JUCE with GPL as default https://github.com/chairaudio/SaM-Designer