Add a headsup to README about filesystem limitations

[johnelm]

• can't access the file system, and modules that depend on fs access may have issues
• browserify won't automatically include modules whose requireing is derived at runtime

why:

• browserify built for browser, which has obvious security constraints
• OSA has its own sandboxing constraints
  • (need to check on accessing filesystem outside sandbox folder using ObjC)

[johnelm]

some good ideas (and browserify alternatives): https://github.com/JXA-Cookbook/JXA-Cookbook/issues/24