Guide for contributors

[lofcz]

Quick symbol explanation for new contributors

No symbol -> Not ported yet
❌ -> Won't be ported, excluded from sgml
✔️ -> Ported
✅ -> Ported, documented
☑️ -> Ported, documented, xml documented
💡 -> New function to sgml (not in base gml)
🔸 -> Ported under a different name
📌-> Ported with extended/changed functionality, eg. params were changed
✒️ -> New function to be added (not implemented yet)