Improve documentation for each shelf

[LuisAlejandro]

We can improve documentation by providing use cases about each shelf. Some suggestions:

• debian: https://github.com/docker-library/docs/blob/master/debian/README.md
• latex: https://github.com/blang/latex-docker/blob/master/README.md
• odoo: https://github.com/docker-library/docs/blob/master/odoo/README.md
• node: https://github.com/docker-library/docs/blob/master/node/README.md
• mongo: https://github.com/docker-library/docs/blob/master/mongo/README.md
• postgres: https://github.com/docker-library/docs/blob/master/postgres/README.md
• python: https://github.com/docker-library/docs/blob/master/python/README.md
• ruby: https://github.com/docker-library/docs/blob/master/ruby/README.md

Also, change README logo to match branding of the language/application.