[mod] #100: document silkaj install from pypi/pip.

doc/install_pip.md

+# Install Silkaj with Pip
+
+## Install dependencies
+
+```bash
+sudo apt install python3-pip libssl-dev
+```
+
+On Ubuntu 14.04 and Debian 8, you need this package too:
+```bash
+sudo apt install libffi-dev
+```
+
+## Install from Pypi
+
+```bash
+pip3 install silkaj
+```
+
+## Install from sources
+
+### Retrieve silkaj sources
+```bash
+sudo apt install git
+git clone https://git.duniter.org/clients/python/silkaj.git
+cd silkaj
+```
+
+### Install with dependencies
+
+Just install:
+```bash
+pip3 install .
+```
+
+Or install it as "editable", for development:
+```bash
+pip3 install -e .
+```
+