Add basic documentation for libkiwix.

It follows the same logic of libzim documentation.

docs/usage.rst

@@ -0,0 +1,17 @@
+Libkiwix programming
+====================
+
+Introduction
+------------
+
+libkiwix is written in C++. To use the library, you need the include files of libkiwix have
+to link against libzim.
+
+Errors are handled with exceptions. When something goes wrong, libzim throws an error,
+which is always derived from std::exception.
+
+All classes are defined in the namespace kiwix.
+
+libkiwix is a set of tools to manage zim files and provide some common functionnality.
+While libkiwix has some wrappers around libzim classes, they are deprecated and will be removed
+in the future.