Update README.md (#2694)
Update README.md (#2694)

## Developer information is hard to find

I've been looking for instructions on how to build gitbucket a couple of times, and I've never found them until takzoe pointed me towards `doc/build.md`. After knowing about this, I read through the readme once again, looking for such a link, and then I've found it as the last sentence within a section named `Features`.

What I've been looking for:
 - either a file named `INSTALL.md`, `BUILD.md` in `/`
 - or a section (or a link) in the main readme

I've put the link to `doc/readme.md` into a separate section "Building and Development" and added a short summary. I'm not so sure about the order of the new section and "Plugins".
xuwei-k/gitbucket,gitbucket/gitbucket,takezoe/gitbucket,imeszaros/gitbucket,takezoe/gitbucket,xuwei-k/gitbucket,xuwei-k/gitbucket,xuwei-k/gitbucket,gitbucket/gitbucket,imeszaros/gitbucket,gitbucket/gitbucket,imeszaros/gitbucket,gitbucket/gitbucket,imeszaros/gitbucket,takezoe/gitbucket,takezoe/gitbucket