docs(overmind-devtools): document overmind-devtools for easier local development

packages/overmind-devtools-client/README.md

@@ -1,3 +1,21 @@
-# overmind-devtools
+### Introduction
 
-To be filled
+There are three packages related to `overmind-devtools` experience:
+* [overmind-devtools-client](../overmind-devtools-client/) - "core" package that provides UI ("web client")
+* [overmind-devtools](../overmind-devtools/README.md) - standalone, electron application
+* [overmind-devtools-vscode](../overmind-devtools-vscode/) - VSCode extension
+
+`overmind-devtools-client` is a core application that can be hosted by available "hosts": `overmind-devtools` and `overmind-devtools-vscode`.
+
+![overmind-devtools-packages](./docs/assets/overmind-devtools-packages.png)
+
+This means that this package (a reminder: we're now in `overmind-devtools-client` package of the monorepo) is **crucial** for the other two.
+
+In case you want to improve devtools experience (UI, UX, etc.), focus on this package specifically.
+
+### Local development
+This package bundle (`bundle.js`) is further used by the companion "host" packages (mentioned before).
+
+Hence, make sure you've build this package before development.
+
+**HINT**: the easiest way to start local development is to run [overmind-devtools](../overmind-devtools/README.md) electron app.