Add build instruction in README, reorganize some sections (#3257)

* Add build instruction in README, reorganize some sections

* Add link to https://docs.gitea.io/en-us/install-from-source/

README.md

@@ -31,13 +31,31 @@ This project has been
 [forked](https://blog.gitea.io/2016/12/welcome-to-gitea/) from
 [Gogs](https://gogs.io) since 2016.11 but changed a lot.
 
-## Notes
+## Building
+
+From the root of the source tree, run:
+
+    make generate all
+
+More info: https://docs.gitea.io/en-us/install-from-source/
+
+## Using
+
+    ./gitea web
+
+NOTE: If you're interested in using our APIs, we have experimental
+support with [documentation](https://godoc.org/code.gitea.io/sdk/gitea).
+
+## Contributing
+
+Expected workflow is: Fork -> Patch -> Push -> Pull Request
+
+NOTES:
 
 1. **YOU MUST READ THE [CONTRIBUTORS GUIDE](CONTRIBUTING.md) BEFORE STARTING TO WORK ON A PULL REQUEST.**
 2. If you have found a vulnerability in the project, please write privately to **security@gitea.io**. Thanks!
-3. If you're interested in using our APIs, we have experimental support with [documentation](https://godoc.org/code.gitea.io/sdk/gitea).
 
-## Docs
+## Further information
 
 For more information and instructions about how to install Gitea, please look
 at our [documentation](https://docs.gitea.io/en-us/). If you have questions
@@ -46,10 +64,6 @@ our [Discord server](https://discord.gg/NsatcWJ),
 [Matrix room](https://matrix.to/#/#gitea:matrix.org),
 or [forum](https://discourse.gitea.io/)!
 
-## Contributing
-
-Fork -> Patch -> Push -> Pull Request
-
 ## Authors
 
 * [Maintainers](https://github.com/orgs/go-gitea/people)