Library submission docs: clarify `npmFilePath`

[SableRaf]

Problem

In contributing_libraries.md, the documentation for the optional npmFilePath field is a bit confusing:

(Optional) npmFilePath: A path like 'dist/library.min.js' if a specific file in the library should be used from npm. You can test this out by going to https://cdn.jsdelivr.net/npm/packageName -- if that doesn't work, try https://cdn.jsdelivr.net/npm/packageName/path/to/file.js and put the path you added into this property

When I followed these instructions while submitting a library, I wasn't sure:

• when npmFilePath is actually needed
• how I was supposed to discover the correct relative path

Suggested improvement

• Explain first that most libraries do not need npmFilePath.

• Provide simple step-by-step instructions:

  1. Check whether https://cdn.jsdelivr.net/npm/packageName loads the library. If it does, you're all set.
  2. If it doesn't, browse https://www.jsdelivr.com/package/npm/packageName to locate the JavaScript file and use its relative path as npmFilePath.