Update THEMES.md regarding custom syntax themes #2573
Description
Update THEMES.md:
- Config syntax (already in Add doc for syntax in THEMES.md #2570)
- Reference to syntect default themes list (already in Add doc for syntax in THEMES.md #2570)
- Custom themes:
- Where to place them
- What format they are
- Maybe a link to https://github.com/filmgirl/TextMate-Themes?