docs: update README

DEVELOPMENT.md

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+## Development
+
+**Requirements:**
+
+- Current version of stable Rust
+- [`just`](https://github.com/casey/just) task runner
+- [`nextest`](https://nexte.st) test runner
+- [`pre-commit`](https://pre-commit.com/)
+- yq (YAML processor)
+
+### Tasks
+
+**Testing**
+
+- `just test` Run unit+integration tests
+- `just unittest` Run unit tests
+- `just testyt` Run YouTube integration tests
+- `just testintl` Run YouTube integration tests for all supported languages (this takes
+  a long time and is therefore not run in CI)
+- `YT_LANG=de just testyt` Run YouTube integration tests for a specific language
+
+**Tools**
+
+- `just testfiles` Download missing testfiles for unit tests
+- `just report2yaml` Convert RustyPipe reports into a more readable yaml format
+  (requires `yq`)