CONTRIBUTING: streamline token guidelines (#1161)
CONTRIBUTING: streamline token guidelines (#1161)

- moved one entry from the general guidelines, since now there's a specific section for tokens;
- reordered and numbered the entries, for clarity and ease of reference
- rewrote some passages for clarity and conciseness.
deekim/tldr,Phizho/tldr,felixonmars/tldr,zoisom/tldr,katekyan/tldr,zoisom/tldr,slash3b/tldr,Phizho/tldr,cptdeka10/tldr,felixonmars/tldr,felixonmars/tldr,deekim/tldr,UncommonPleb/tldr,cptdeka10/tldr,Phizho/tldr,deekim/tldr,zoisom/tldr,katekyan/tldr,UncommonPleb/tldr,slash3b/tldr,UncommonPleb/tldr,cptdeka10/tldr,katekyan/tldr,slash3b/tldr