See here.

I like this for several reasons:

• I like the simplicity and clarity of the guidelines.
• I agree with all of their guidelines; nothing feels controversial there. Such as: Use JSON, and pretty print it. Be explicit with error messages. Use plural nouns for containers. Etc.
• I like the fact that it is open sourced for the world to see, share and fork.

ps: My employer, Apigee, is still looking to hire SEs, and other API geeks.