README.md should include link to documentation

**Is your feature request related to a problem? Please describe.**

When I was getting started I looked for the website with the documentation on the main page but couldn't find it. I had to google it to find https://drogon.org/ and https://drogonframework.github.io/drogon-docs. I was still confused thinking one of them was unmaintained until I noticed the "docs" link in the drogon.org navbar links to the same https://drogonframework.github.io/drogon-docs.

**Describe the solution you'd like**

The website would be included on the main page and on the project sidebar. A person searching (ctrl + f) for "documentation" would find it.

**Describe alternatives you've considered**

Googling it, which is suboptimal/incompatible for a project as good as this.