Update development policy to relax rules on committer agreements

[brson]

There has been discussion lately about whether our development policy is too strict regarding committer agreements. Specifically, we require them for all patches, which is not the norm for Mozilla.

There seems to be agreement that the committer agreement only applies to those with commit access to the main repository and that they take on the liability for anything they push. We should update the development policy on the wiki to reflect this.

[brson]

I suppose this warrants a post to rust-dev as well, to make sure everyone with commit access knows about it.

[brson]

Just a note that I pulled a commit from Haitao Li despite the development policy not being updated yet.

[brson]

Now we've pulled from a number of contributors without asking for an agreement.

[brson]

I have gone ahead and removed the requirement for committer agreements from the development policy.