Add note that NODE_PATH is not supported by browserify

[sternenseemann]

browserify doesn't implicitly set opts.paths to NODE_PATH anymore unlike stated previously in this documentation. For background see:

• https://github.com/browserify/resolve/issues/39#issuecomment-306223854
• https://github.com/browserify/resolve/pull/47
• https://github.com/browserify/browserify/issues/1626