Added installation guide

INSTALL.md

@@ -0,0 +1,52 @@
+##Installation
+
+###Linux:
+
+You should get and install [libsodium](https://github.com/jedisct1/libsodium):
+```bash
+git clone git://github.com/jedisct1/libsodium.git
+cd libsodium
+git checkout tags/0.4.2
+./autogen.sh
+./configure && make check
+sudo make install
+sudo ldconfig
+```
+
+Then clone this repo and run:
+```bash
+cmake CMakeLists.txt
+```
+
+Then you can build any of the [`/testing`](/testing) and [`/other`](/other) by running:
+```bash
+make name_of_c_file
+```
+For example, to build [`Messenger_test.c`](/others/Messenger_test.c) you would run:
+```bash
+make Messenger_test
+```
+
+###Windows:
+
+You should install:
+  - [MinGW](http://sourceforge.net/projects/mingw/)'s C compiler
+  - [CMake](http://www.cmake.org/cmake/resources/software.html)
+
+Then you should either clone this repo by using git, or just download a [zip of current Master branch](https://github.com/irungentoo/ProjectTox-Core/archive/master.zip) and extract it somewhere.
+
+After that you should get precompiled packages of libsodium from [here](https://download.libsodium.org/libsodium/releases/) and extract the archive into this repo's root. That is, `sodium` folder should be along with `core`, `testing` and other folders.
+
+Navigate in `cmd` to this repo and run:
+```cmd
+cmake -G "MinGW Makefiles" CMakeLists.txt
+```
+
+Then you can build any of the [`/testing`](/testing) and [`/other`](/other) by running:
+```cmd
+mingw32-make name_of_c_file
+```
+For example, to build [`Messenger_test.c`](/others/Messenger_test.c) you would run:
+```cmd
+mingw32-make Messenger_test
+```