[doc] #142: Pip install: add dependency for Linux Mint

doc/install_pip.md

@@ -6,11 +6,18 @@
 sudo apt install python3-pip libssl-dev
 ```
 
-On Ubuntu 14.04 and Debian 8, you need this package too:
+On Ubuntu (14.04 and 16.04) and Debian 8, you need this package too:
 ```bash
 sudo apt install libffi-dev
 ```
 
+Your system must use UTF-8 locales.
+
+Linux Mint is reported to require the installation of this package too:
+```
+sudo apt install python3-dev
+```
+
 ## Install from Pypi
 
 ```bash