My guidelines are:
Document in english
Document as minimum as possible, documentation is hard to maintain, if you can document it with the test suite, just do it!
(if available put here badges like quality, build)
Add here a brief description of the project, what it’s intent, what kind of problems it solves, what kinds it doesn’t. Add here too if he has a tight dependency at another project.
Add here all the tools need to code and build the project on a developers machine
Add here the process taken to build the project, please give preference to command line tools.
If needed add here a deploy model
Add here what dependencies are needed to run the project on a developer’s machine and how to run the artifacts described at deploy model, please give preference to command line tools.
Describe here how to run integrated tests.