[!340] Add documentation about how to use dune-codegen in an offline environment

Merge branch 'feature/offline-usage' into 'master'

ref:extensions/dune-codegen @rhess Could you try this at some point.

The docs are currently missing how a local package index can be relocated into
a completely isolated environment, but I want to wait until the [devpi
website] is up again to read up on that.

This fixes [#127].

See merge request [extensions/dune-codegen!340]

  [devpi website]: https://doc.devpi.net/
  [#127]: gitlab.dune-project.org/NoneNone/issues/127
  [extensions/dune-codegen!340]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/340


Closes #127

README.md

@@ -105,6 +105,25 @@ ctest
 
 Note that this takes quite a while.
 
+## Building and Running dune-codegen in an offline environment
+
+dune-codegen relies on installing Python packages into self-contained environments
+during its configuration and build process. In order to do this in an offline
+environment, we recommend using the tool `devpi`. One of its use cases is to provide
+a local mirror for the Python package index. A quickstart tutorial for this use case
+is available [5]. It boils down to the following:
+
+* Installing the `devpi-server` package through your favorite method
+* Setting up a local server with `devpi-server --init`
+* Making sure it is running in the background (explicitly with `devpi-server --start/stop` or by configuring a systemd service.
+* Have the environment variable `PIP_INDEX_URL` to its index, e.g. by adding this line to your `~/.bashrc` (where `http://localhost:3141` might differ depending on your devpi configuration):
+```
+export PIP_INDEX_URL=http://localhost:3141/root/pypi/+simple/
+```
+
+At first installation, the locally mirrored package index will access PyPI.
+Later on, it will install packages from its local cache.
+
 ## Links
 
 [0]: https://git-lfs.github.com/
@@ -112,3 +131,4 @@ Note that this takes quite a while.
 [2]: https://gitlab.dune-project.org/quality/dune-testtools
 [3]: http://isl.gforge.inria.fr/
 [4]: https://www.dune-project.org/doc/installation/
+[5]: https://github.com/devpi/devpi/blob/master/doc/quickstart-pypimirror.rst