Update README instructions

[AlekSi]

I generated Go words for Go 1.9: https://github.com/AlekSi/go-words/blob/master/go.txt

I wanted to contribute them but looks like README is outdated. It says:

The source text for each dictionary is in ./server/dictionaries.

But this directory does not exist.

Use the npm run build-dictionaries command to build the dictionaries for the extension.

But this script does not exist.

[AlekSi]

Ok, I found it – in the other repository in another account. 😕

[Jason3S]

Thank you. The documentation is out of date.

I have been slowly moving all the dictionaries to cspell-dicts. The idea is to make it easy to add more dictionaries and extension add ons.

[IgorKrupenja]

Additionally, suggest adding a link to the https://github.com/streetsidesoftware/vscode-cspell-dict-extensions repo somewhere under Add-On Language Dictionaries. It took me ~20 minutes to figure out where/how to create a PR for a new dictionary extension. If OK, I can make a PR for this Readme change.