ngircd/README.md: Document the 20-character password limit

From the referenced INSTALL docs:

  Differences to version 0.8.x

  - The maximum length of passwords has been raised to 20 characters
    (instead of 8 characters). If your passwords are longer than 8
    characters then they are cut at an other position now.

so in v0.8 and earlier the password limit was even shorter.