Update docs.

Signed-off-by: Rolf Schröder <[email protected]>

README.rst

@@ -162,6 +162,8 @@ There are several options to use the Nix version
 
 The Nix installation uses `mach-nix <https://github.com/DavHau/mach-nix>`__ to handle Python dependencies because some dependencies are currently not available as Nix packages.
 All Python dependencies are automatically fetched from ``./requirements.txt``.
+If the ``mach-nix``-based installation fails, you might need to update ``mach-nix`` itself and the `pypi-deps-db <https://github.com/DavHau/pypi-deps-db>`_ version in use (see ``etc/nix/flake.nix:inputs.machnix`` and ``machnixFor.pypiDataRev``).
+
 Non-Python dependencies are curated in ``etc/nix/flake.nix:vulnerablecode.propagatedBuildInputs``.
 
 
@@ -260,4 +262,4 @@ For full documentation about API endpoints.
 .. |Gitter chat| image:: https://badges.gitter.im/gitterHQ/gitter.png
    :target: https://gitter.im/aboutcode-org/vulnerablecode
 .. |PRs Welcome| image:: https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square
-   :target: http://makeapullrequest.com
+   :target: http://makeapullrequest.com

etc/nix/flake.nix

@@ -49,7 +49,9 @@
           python = "python38";
 
           # Pin pypi repo to a specific commit which includes all necessary
-          # Python deps. The corresponding sha256 hash can be obtained with:
+          # Python deps. The default version is updated with every mach-nix
+          # release might be be sufficient for newer releases.
+          # The corresponding sha256 hash can be obtained with:
           # $ nix-prefetch-url --unpack https://github.com/DavHau/pypi-deps-db/tarball/<pypiDataRev>
           pypiDataRev = "c86b4490a7d838bd54a2d82730455e96c6e4eb14";
           pypiDataSha256 =