From Tom Preston-Werner's article,
Write your Readme first.
Create a README.md in your repository root folder and write there all information needed to setup, install, develop the related software. Include also links and references to external sources but try to keep the information auto consistent as much as possible, that is
Keep in your Readme everything you need to know quickly about the project.
Consider put a README.md also in sub folders (for instance, like this file) where it is needed, cause GitHub site will render them.
Markdown is easy to use and has great benefits, use it to document your project. Take a look to Mastering Markdown GitHub Guide.
Learn how to use git branches, try to avoid working on master branch. Read the this short branch tutorial.
Use EditorConfig to mantain a consistent coding style between different editors. To get started, add an .editorconfig at the root of your repo with the following sample content
root = true
[*]
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
[*.{js,json}]
indent_style = space
indent_size = 2
[*.{sql}]
indent_style = tab
Check here if your editor supports EditorConfig natively (for example WebStorm) or you need to install a plugin (for example editorconfig-vim or editorconfig-sublime).
Find last version of this document here.