README: document dependencies and Makefile targets

Add a Development section explaining:
- How to install ruff (from snap) and mypy (from apt) on Debian/Ubuntu
- All available Makefile targets with descriptions
- Quick start commands for linting and building

Signed-off-by: Igor Opaniuk <igor.opaniuk@oss.qualcomm.com>

Author: igoropaniuk

README.md

@@ -7,6 +7,48 @@ qcom-ptool contains various device partitioning utilities like ptool.py, gen_par
 
 # Development
 
+## Dependencies
+
+The scripts use only the Python standard library (Python 3.8+), so no
+runtime dependencies need to be installed.
+
+For development, `make lint` invokes `ruff` and `mypy` directly from the
+command line. On Debian/Ubuntu, install them as follows (ruff is not
+packaged in apt on all releases/architectures, so we install it from
+snap):
+
+```sh
+sudo snap install ruff
+sudo apt install mypy
+```
+
+## Makefile targets
+
+| Target        | Description                                                |
+|---------------|------------------------------------------------------------|
+| `all`         | Generate partition XML and GPT binaries for all platforms  |
+| `lint`        | Run ruff (linter) and mypy (type checker) on all scripts   |
+| `integration` | Build all platforms and verify generated files are present |
+| `check`       | Run both `lint` and `integration`                          |
+| `install`     | Install scripts to `$(DESTDIR)$(PREFIX)/bin`               |
+| `clean`       | Remove generated XML and binary files from platforms/      |
+
+### Quick start
+
+```sh
+# install linters (Debian/Ubuntu)
+sudo snap install ruff
+sudo apt install mypy
+
+# run linters
+make lint
+
+# build all platforms and run tests
+make check
+```
+
+## Code contributions
+
 See [CONTRIBUTING.md file](CONTRIBUTING.md) for instructions on how to send
 code contributions to this project. You can also [report an issue on
 GitHub](../../issues).