update readme

Author: jodersky

README.md

@@ -2,7 +2,8 @@
 
 [![project docs](https://img.shields.io/badge/docs-website-blueviolet)](https://jodersky.github.io/scala-argparse)
 [![project chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://crashbox.zulipchat.com/#narrow/stream/330744-argparse)
-[![Latest version][scaladex-badge]][scaladex-link]
+[![latest version][scaladex-badge]][scaladex-link]
+[![stability: firm](https://img.shields.io/badge/stability-firm-silver)](https://www.crashbox.io/stability.html)
 
 [scaladex-badge]: https://index.scala-lang.org/jodersky/scala-argparse/argparse/latest.svg
 [scaladex-link]: https://index.scala-lang.org/jodersky/scala-argparse/argparse
@@ -26,5 +27,24 @@ Pragmatic command line parsing for Scala applications.
 
 ## Docs
 
-Look at the [website](https://jodersky.github.io/scala-argparse), or,
-equivalently, browse the docs/ folder.
+- HTML: look at the [website](https://jodersky.github.io/scala-argparse)
+- source (markdown): browse the docs/ folder.
+
+## Building
+
+This project uses [Mill](https://github.com/com-lihaoyi/mill) to build. The
+configuration is in the `build.sc` file.
+
+### Developer
+
+- compile main project for all supported versions of scala: `./mill argparse.__.compile`
+- run all tests: `./mill __.test`
+- run an example:
+  - `./mill examples.<name of example> <arguments>`
+  - e.g. `./mill examples.paramnamed --verbosity 5`
+  - note: examples use Scala Native and hence require llvm
+- publish main project locally: `./mill argparse.__.publishLocal`
+
+### Maintainer
+
+Look at the scripts in the `ci/` directory.