Add Apache licence headers to non-code files

`CONTRIBUTING.md` requires Apache 2.0 licence headers on all committed files, but several non-code files currently lack them:

- `geonames/terms.txt`
- `geonames/files.txt`
- `nyc_taxis/files.txt`
- `geonames/configsets/geonames/stopwords.txt`
- `geonames/configsets/geonames/synonyms.txt`
- `nyc_taxis/configsets/nyc_taxis/stopwords.txt`
- `nyc_taxis/configsets/nyc_taxis/synonyms.txt`

Add the standard ASF licence header comment block to each file. Consider adding `apache-rat` to the CI workflow (see #14) to enforce this automatically going forward.

Start with CONTRIBUTING.md to find the required standard ASF Apache 2.0 header, then inspect the seven listed .txt files for their current contents and comment conventions. Add the header to each named file and verify that all seven contain it; apache-rat enforcement is mentioned separately in #14.

Written by the indexing model from the issue text.