Anyone may contribute to Steep in any way they like, however any contributions will be merged on a case by case basis. When contributing its best to get in contact with the owners by posting an issue regarding the change.
There is no code of conduct, but that is not permission to act however you please. Don't be a dick.
The plugin is being developed with Sublime Text, but you may develop with whatever you like. The project includes a sublime project file if you wish to use it.
As stated, any contributions will be merged on a case by case basis. Be sure to make any changes clear and distinct, or better, discuss the proposed changes prior to making a pull request. Not all pull requests will be merged.