README: add python ecdsa package

Add ecdsa to the pip install command. This package is needed since Golioth
Firmware SDK v0.16.0 (probably part of NCS v2.8.0 but unconfirmed).
Unfortunately the Zephyr requirements.txt does not include this package.

Signed-off-by: Mike Szczys <[email protected]>

Author: szczys

README.rst

@@ -31,7 +31,7 @@ Install the Python virtual environment (recommended)
    mkdir golioth-reference-design-template
    python -m venv golioth-reference-design-template/.venv
    source golioth-reference-design-template/.venv/bin/activate
-   pip install wheel west
+   pip install wheel west ecdsa
 
 Use ``west`` to initialize and install
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