docs: update dependency install instructions (Sinaptik-AI#186)

Running `poetry install` does not install all dependencies required in tests.
Update instructions in CONTRIBUTING.md to include `--all-extras` flag.
Closes Sinaptik-AI#182

CONTRIBUTING.md

@@ -23,7 +23,7 @@ We use `poetry` as our package manager. You can install poetry by following the
 Please DO NOT use pip or conda to install the dependencies. Instead, use poetry:
 
 ```bash
-poetry install
+poetry install --all-extras
 ```
 
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