Our [contrinute page](https://sql-machine-learning.github.io/doc_index/contribute.html) should at least include instructions of building and testing the project, aka [build.md](https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/build.md). Update: My bad. `build.md` is under "Canonical Development Environment". Shall we change it to a more obvious name like "How to Build and Test"?