Document how to add words to dictionaries

[AndiDog]

Contributors should be able to find in CONTRIBUTING.md how common words per language can be added. If that's not easy, people will keep having to add lots of words to their local dictionary which does not help other users. I looked around for a few minutes but could not easily find this information.

[Jason3S]

@AndiDog,

You make a good point. Would you be open to making a PR that points would be contributors to streetsidesoftware/cspell-dicts: Various cspell dictionaries? All the extensions in this repo are based upon the dictionaries found in cspell-dicts.

I must admit, the instructions on how to add words is not ideal. Most dictionaries have a src/additional_words.txt file.