docs: explain how to handle read-only sandbox paths

[AlexTereshenkov]

Running build without --sandbox_writable_path=/path/to/cache might result in error: unable to create compilation: ReadOnlyFileSystem which has been reported in Bazel Slack workspace. I have also personally experienced this in a small project of mine.

I thought adding this to the docs might help the next user reading it should they face with the same issue.

[CLAassistant]

All committers have signed the CLA.