[enh] #23: doc: provide virtual env installation process.

src/doc/install_pyenv.md

+# Install Silkaj on a Python environment
+
+## Install Pyenv
+
+### Retrieve pyenv sources
+```bash
+git clone https://github.com/pyenv/pyenv.git ~/.pyenv
+```
+
+### Handle shell modifications: point 2,3 and 4.
+- [Follow pyenv install documentation](https://github.com/pyenv/pyenv#installation)
+
+
+### Install latest Python version and create pyenv
+```bash
+pyenv install 3.6.0
+pyenv shell 3.6.0
+pyenv virtualenv silkaj-env
+```
+
+## Install Silkaj
+
+### Retrieve silkaj sources
+```bash
+git clone https://github.com/duniter/silkaj.git
+cd silkaj
+```
+
+### Install dependencies and store them on pyenv environement
+```bash
+pip install -r requirements.txt --upgrade
+pyenv rehash
+```
+
+### Activate pyenv and run silkaj
+```bash
+pyenv activate silkaj-env
+./silkaj
+```