Update README.md with basic documentation on installation and functionalities of this extension

[trnstlntk]

By end October it would be good to update this repo's README.md so that it becomes clear for laypeople who want to install and use the extension. Basic things to include:

• Info on how to install the extension (can link to more specific info in our docs, but let's have some basic info here too)
• Info and a few examples on the GREL commands we built (same)
• Info and examples of the workflow of starting a project with Commons categories (same)

[trnstlntk]

Notes to self

• GREL for extracting Description parameter of Information template of a Wikitext column: do Edit column... > Add column based on this column... > extractFromTemplate(value, "Information", "Description")
• GREL for extracting categories from a Wikitext column: do Edit column... > Add column based on this column... > value.extractCategories()

[wetneb]

For your example GREL formulae above, remember that they generate arrays and that you ran into OpenRefine/OpenRefine#1475 and OpenRefine/OpenRefine#1088 when using this yourself - this definitely requires documenting

[trnstlntk]

I started to write about the intricacies of the array issue in the GREL, but then decided against having that in a simple how-to readme (it will be distracting; I think it's better to just present the user with one very easy replicable example that works). OpenRefine's user manual is a better place to document that array issue IMO.