Adopt a Code of Conduct

Merge branch 'add-postgres-official' of https://github.com/jwworth/free-programming-books into jwworth-add-postgres-official

Update free-programming-books-zh.md

Update free-programming-books-zh.md

Update free-programming-books-pl.md

mongodb book

I think it's a shame that the second most popular github repository does not have a code of conduct and I intend to fix this. We never needed a code of conduct, we don't want to need a code of conduct and that's why we need a code of conduct.

I submit this as a PR instead of commiting directly to master because I want to allow some debate on this issue. I will merge this PR in a week or so, if there is no good case against it.

html5 book

Fix and add Japanese scala contents

Add A Brief Introduction to Neural Networks