HITRAN readme.txt should be renamed

The file astroquery/hitran/data/readme.txt is actively used by the hitran module.
In my opinion, it should be renamed to avoid confusion with real documentation files.

Inspect astroquery/hitran/data/readme.txt and search the hitran module for every reference to it. Confirm the intended replacement name with maintainers, update all references, and verify that the HITRAN module still accesses the file successfully after the rename.

Written by the indexing model from the issue text.