Document `GRADLE_PROFILER_OPTS`

[ianbrandt]

I just hit an OutOfMemoryError in the Gradle Profiler main thread. From the generated heap dump I see the max heap defaults to -Xmx64m. Looking at the source of the startup scripts it appears I can use the GRADLE_PROFILER_OPTS environment variable to configure a higher max heap.

It would be handy if GRADLE_PROFILER_OPTS was documented in the README.

[darksapien23151]

@ianbrandt would be glad if I could be assigned with the issue