Add README with motivation

examples/v1-pip-compile/README.md

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+## Motivation
+
+A project consists of multiple python packages in different subdirectories. Each
+of these has dependencies and optional dev dependencies. Compiled requirements
+with hashes exist, written with pip-compile:
+
+- ``code1/pypkg1/requirements.txt``
+- ``code1/pypkg1/requirements-dev.txt``
+
+Two modes of operation:
+
+1. Use compiled requirements files and create nix lock files accordingly.
+2. Create nix lock files without using compiled requirements, but only
+   dependency declarations in pyproject.toml, and pip-compile afterwards for
+   projects not using nix.
+
+We'd like to create environments where requirements for individual packages and
+combinations of these packages are available.
+
+For the projects' python packages:
+
+1. Do not install, environment with requirements only.
+
+2. Install selected packages.
+
+3. Install selected packages in editable mode.