Add note about empty path segments to README

[balazsbotond]

See this Reddit comment: https://www.reddit.com/r/programming/comments/iyh2yn/urlcat_build_correct_urls_easily/g6ebzjr?utm_source=share&utm_medium=web2x&context=3

[balazsbotond]

The README section should include:

• we support empty path segments (with an example of what they are and a reference to the RFC that allows them)
• though duplicate slashes are automatically removed from the concatenation boundary
• so if you need an empty segment at the bondary, you should either use:
  • this overload: urlcat(baseTemplate: string, params: ParamMap) (with usage example)
  • one of the other two overloads but put an extra slash at the beginning of pathTemplate (with usage example)

This section shouldn't be prominent in the README because it's a very rare edge case.