Remove style guide and code of conduct and add developer guidelines submodule

[jasonklein]

The developer-guidelines repo obviates the need to have individual copies of the Solidity Style Guide and Code of Conduct in this repository. However, this repository should keep up with those materials.

Consequently, those files should be removed and the developer-guidelines repo added as a git submodule. Additionally, the readme for this repo should be updated to link to those items.

[benjaminbollen]

discuss during planning poker whether the guidelines need to be submodule-ed in. The README.md links should still link to the github.com/openst/developer-guidelines regardless.