Add Apache licence headers to non-code files

[janhoy]

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.