Added README with local development instructions

README.md

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+# COSMIC Applets Development
+
+This repository is a Rust workspace for COSMIC panel applets and panel buttons.
+
+## Install Build Dependencies
+
+On Debian or Ubuntu
+
+```bash
+sudo apt build-dep .
+```
+
+## Getting Started
+
+
+A [justfile](./justfile) is included by default with common recipes used by other COSMIC projects. Install from [casey/just][just]
+
+- `just` builds the application with the default `just build-release` recipe
+- `sudo just install` installs the project into the system
+- `just vendor` creates a vendored tarball
+- `just build-vendored` compiles with vendored dependencies from that tarball
+- `just check` runs clippy on the project to check for linter warnings
+- `just check-json` can be used by IDEs that support LSP
+
+
+## Reload The Current Applets
+
+After installing a new build, restart the panel so COSMIC relaunches the applets:
+
+```bash
+systemctl --user restart cosmic-session.target
+pkill cosmic-panel
+```
+
+You can also restore the packaged version by reinstalling the distro package:
+
+```bash
+sudo apt install --reinstall cosmic-applets
+pkill cosmic-panel
+```