.github: add CONTRIBUTING.md (#17476)

The contributing instructions in the README are not in the GitHub contributing
guide, which means that people coming from the GitHub issues are less likely to
see them.

.github/CONTRIBUTING.md

+# Contributing
+
+Thank you for considering to help out with the source code! We welcome 
+contributions from anyone on the internet, and are grateful for even the 
+smallest of fixes!
+
+If you'd like to contribute to go-ethereum, please fork, fix, commit and send a 
+pull request for the maintainers to review and merge into the main code base. If
+you wish to submit more complex changes though, please check up with the core 
+devs first on [our gitter channel](https://gitter.im/ethereum/go-ethereum) to 
+ensure those changes are in line with the general philosophy of the project 
+and/or get some early feedback which can make both your efforts much lighter as
+well as our review and merge procedures quick and simple.
+
+## Coding guidelines
+
+Please make sure your contributions adhere to our coding guidelines:
+
+ * Code must adhere to the official Go 
+[formatting](https://golang.org/doc/effective_go.html#formatting) guidelines 
+(i.e. uses [gofmt](https://golang.org/cmd/gofmt/)).
+ * Code must be documented adhering to the official Go 
+[commentary](https://golang.org/doc/effective_go.html#commentary) guidelines.
+ * Pull requests need to be based on and opened against the `master` branch.
+ * Commit messages should be prefixed with the package(s) they modify.
+   * E.g. "eth, rpc: make trace configs optional"
+
 ## Can I have feature X
 
-Before you do a feature request please check and make sure that it isn't possible
-through some other means. The JavaScript enabled console is a powerful feature
-in the right hands. Please check our [Wiki page](https://github.com/ethereum/go-ethereum/wiki) for more info
+Before you submit a feature request, please check and make sure that it isn't 
+possible through some other means. The JavaScript-enabled console is a powerful 
+feature in the right hands. Please check our 
+[Wiki page](https://github.com/ethereum/go-ethereum/wiki) for more info
 and help.
 
-## Contributing
-
-If you'd like to contribute to go-ethereum please fork, fix, commit and
-send a pull request. Commits which do not comply with the coding standards
-are ignored (use gofmt!).
+## Configuration, dependencies, and tests
 
-See [Developers' Guide](https://github.com/ethereum/go-ethereum/wiki/Developers'-Guide)
-for more details on configuring your environment, testing, and
-dependency management.
+Please see the [Developers' Guide](https://github.com/ethereum/go-ethereum/wiki/Developers'-Guide)
+for more details on configuring your environment, managing project dependencies
+and testing procedures.