Minor fixes in installation procedure

[glatard]

I just installed CARMIN server in a fresh virtual env on CentOS 7, it went very well, congrats for the great work @louis-ver and @simon-dube!

Minor comments:

• I had to install postgresql-devel to provide pg_config. Maybe it would be worth documenting it.
• I had to pip install typing.
• The need to setup DATABASE_URI should be documented.
• It should be mentioned that Python >= 3.5 is required (I tried with 3.4 and it crashes). It would be useful to support Python 3.4 if possible, Python 3.5 is a pain to install on CentOS7.
• Document how to create a new user.
• Document how to change the admin password.

[simon-dube]

Additional elements to address:

• Add an endpoint to allow users for password modification

[louis-ver]

Add an endpoint to allow users for password modification
Document how to create a new user.
Document how to change the admin password.

Addressed in simon-dube#14,

It should be mentioned that Python >= 3.5 is required (I tried with 3.4 and it crashes). It would be useful to support Python 3.4 if possible, Python 3.5 is a pain to install on CentOS7.
I had to pip install typing.

simon-dube#13,

The need to setup DATABASE_URI should be documented.

simon-dube#11

[louis-ver]

Added in #6