docs [installation]: Add development instructions.

Add some basic development instructions with idiap-devtools.

instructions/development.md

+### Developing a bob package
+
+We follow the development procedure defined in
+[idiap-devtools](https://www.idiap.ch/software/biosignal/docs/software/idiap-devtools/main/sphinx/).
+You will need a working conda installation with `devtool` in it (preferably in `base`),
+installed with:
+
+``` sh
+mamba install -n base -c https://www.idiap.ch/software/bob/conda/label/beta -c conda-forge idiap-devtools
+```
+
+You will also need a local copy of bob's `dev-profile`:
+
+``` sh
+git clone git@gitlab.idiap.ch:bob/dev-profile.git /your/path/to/idiap-profiles/bob
+```
+
+To create a development environment for a particular package execute:
+
+``` sh
+# git clone git@gitlab.idiap.ch:bob/bob.bio.face.git
+# conda activate
+devtool env -P /your/path/to/idiap-profiles/bob ./bob.bio.face
+mamba env create -f environment.yaml -n bob_face_devel
+mamba activate bob_face_devel
+pip install --no-deps -e ./bob.bio.face
+```
+
+> **NOTE:**
+>
+> You can also define the profiles' paths in a configuration file (see here) so you can
+> use the simpler `devtool env` command:
+>
+> ``` sh
+> devtool env -P bob ./bob.bio.face
+> ```