Add a page about contributing datasets

[auxesis]

To make adding new datasets easy for enterprising civic tech hackers, there needs to be a guide that explains how to contribute new scrapers and pull the data into Got Gastro.

Should cover:

• [x] Writing and publishing a scraper on Morph
• [x] Pulling the data into gotgastro_scraper
• [x] What to do in your scraper, vs what to do in gotgastro_scraper
• [x] What the Got Gastro architecture looks like (nested scrapers, gotgastro_scraper, the /reset function, how it interplays with email alerts)
• [x] The PR process
• [x] To geocode, or to not geocode
• [x] When to normalise the data (fixing up dates, multiple addresses)
• [x] Handling complex HTML data (by converting it to Markdown)

[auxesis]

Starting work in feature/document-contributing-datasets.