[nix-ci] Add a README in dev/ci/nix

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+# Working on third-party developments with *this* version of Coq
+
+Aim: getting an environment suitable for working on a third-party development
+using the current version of Coq (i.e., built from the current state of this
+repository).
+
+Dive into such an environment, for the project `example` by running, from the
+root of this repository:
+
+    ./dev/ci/nix/shell example
+
+This will build Coq and the other dependencies of the `example` project, then
+open a shell with all these dependencies available (e.g., `coqtop` is in path).
+
+Additionally, three environment variables are set, to abstract over the
+build-system of that project: `configure`, `make`, and `clean`. Therefore, after
+changing the working directory to the root of the sources of that project, the
+contents of these variables can be evaluated to respectively set-up, build, and
+clean the project.