Document the valid characters in a namespace

[kineticsquid]

The valid characters for a namespace are currently documented only in the source code. Suggest adding the following sentence to this wiki page: https://github.com/eclipse/openvsx/wiki/Namespace-Access

Valid namespace names are represented with the regex [\w\-\+\$~]+. This translates to the letters 'a' - 'z', 'A' - 'Z'; numbers '0' - '9'; and the characters '_', '-', '+', '$' and '~'.

I think it should go after this sentence:

The publisher field in the package.json of VS Code extensions defines a namespace in which the extension is published.