This is where, you guessed it, your rules are going to go. Now, everyone has different rules when it comes to writing, so you need to really think about what you want to put here. Perhaps you have pet peeves that you don't want anyone to cross, perhaps there are some things you particularly like in a writing partner.
Some standard rules can be found here. Check out the "golden rules," as they go over the Top 10 rules of roleplaying that are usually pretty universal across the board. The basics are all there, but the big ones to remember (my personal favorites) are your cliches (no power playing, God-moding, Gary/Mary sues), respect, communication, and to have fun. If writing starts to become a chore... then why are you still doing it?
Aside from those, it might be good to remember that if you're over the age of 18 and want to incorporate adult themes, especially romance and smut, the person you're writing with must be 18+, as well. If you want a good explanation as to why, you can look here. It's a pretty all-encompassing article with good resources. This is a big deal, and not enough people take it seriously.
In addition, you might want to include any limitations you have on this page. If you like to fade to black, now would be the time to mention it. If you swear like a sailor, perhaps you should point that out. If you have the horrible habit of killing off characters in brilliant movie-style technicolor, you might want to let your potential partners know.
Along that line, also think about the types of characters you like to play, and the pairings you prefer. If you're looking at playing only female characters in specifically ONLY male-and-female pairings, you might want to mention that. Also consider whether or not you like to double, and how long your posts are. Indicate if you're okay with writing multiple characters and if you write in paragraphs or pages here.