Add section about external projects in README (#1075)

Author: alexarchambault

README.md

@@ -127,6 +127,29 @@ that embeds JARs of the scala-cli modules, and downloads their dependencies
 from Maven Central upon first launch (using the coursier cache, just like
 a coursier bootstrap).
 
+### Helper projects
+
+A number of features of Scala CLI are managed from external projects, living under
+the [`scala-cli` organization](https://github.com/scala-cli) on GitHub. These
+projects can be used by Scala CLI as libraries pulled before it's compiled, but also
+as binaries. In the latter case, Scala CLI downloads on-the-fly binaries from these
+repositories' GitHub release assets, and runs them as external processes.
+
+For example, here are a few external projects used by Scala CLI:
+- [scala-js-cli-native-image](https://github.com/scala-cli/scala-js-cli-native-image): provides a binary running the Scala.js linker
+- [scala-cli-signing](https://github.com/scala-cli/scala-cli-signing): provides both libraries and binaries to handle PGP concerns in Scala CLI
+- [libsodiumjni](https://github.com/scala-cli/libsodiumjni): provides minimal JNI bindings for
+[libsodium](https://github.com/jedisct1/libsodium), that is used by Scala CLI to encrypt secrets
+uploaded as GitHub repository secrets in the `publish setup` sub-command
+
+For the full list of those projects and their description, see the
+[scala-cli repository list](https://github.com/orgs/scala-cli/repositories) and the READMEs
+of each of these projects.
+
+The use of external binaries allows to make the Scala CLI binary slimmer and faster
+to generate, but also allow to lower memory requirements to generate it (allowing to
+generate these binaries on the GitHub-provided GitHub actions hosts).
+
 ### Website
 
 The Scala CLI website is built with [Docusaurus](https://v1.docusaurus.io/en/) and uses [Infima](https://infima.dev/docs/layout/spacing) for styling.