[Docs] Added a requirements section on the readme. (See #9)

## Requirements

The project need the following libraries to build:

* [glibc](https://www.gnu.org/software/libc/) (>=2.20)

* [openssl](https://www.openssl.org/) (>=1.0.1.l)

* [libxml2](http://www.xmlsoft.org/) (>=2.9.2)

* [libcurl](http://curl.haxx.se/libcurl/) (>=7.40.0)

* [libconfig](http://www.hyperrealm.com/libconfig/) (>=1.5)

* [libbcm2835](http://www.airspayce.com/mikem/bcm2835/) (>=1.46)

The versions describe the lowest tested version. Is it possible that lower

and/or higher versions will run fine, too.

The project was successfully compiled with the [GCC](https://gcc.gnu.org/) (>=4.9.2).

## Installation

### From Source