documentation updates

[tooomm]

• [ ] CONTRIBUTING.md (e.g. code style conventions: https://github.com/Cockatrice/Magic-Spoiler/pull/98#issuecomment-313163870)
• [ ] README.md: part for developer and part for user (e.g. quick links to user editable files)
• [ ] Wiki maybe

[dev-id]

• ~~Document adding new set for new season similar to mtgjson~~

[skwerlman]

• [x] Explain (either in release notes or in the wiki) the difference between spoiler.xml and <setcode>.xml

[tooomm]

Groundwork for correction files documentation layout done in #133

Notes for myself:

• Still missing a hint for copy and paste workflow from error log to corrections file

• manual file: change Form part something like this possible?

  - name attribute: value
    other fields: value

• explanation comments: (additionally? instead?) link to wiki with more details and explanations of all keys etc.

• important key fields are: name, manaCost (calculation for cmc and color) and type to make subtype, supertype, and check to see if a card should have certain properties (loyalty for PW, p/t for creature), rarity and url also very important for draft page (and information on booster in set_info)

[dev-id]

• [x] CONTRIBUTING.md has some remaining json references

[tooomm]

• [x] Explain (either in release notes or in the wiki) the difference between spoiler.xml and <setcode>.xml

We have it in the README now: https://github.com/Cockatrice/Magic-Spoiler#output

• [x] CONTRIBUTING.md has some remaining json references

The old references to the former correction json files are gone, we don't use them as the script fundamentally changed in between and utilizes the Scryfall API after a rewrite: https://github.com/Cockatrice/Magic-Spoiler/blob/master/.github/CONTRIBUTING.md