docs : add explanation of what is unisync

also add an example

docs/source/example.rst

@@ -0,0 +1,24 @@
+.. _example_how_it_works:
+
+Example of how unisync works
+============================
+
+Let's say you have the following structure::
+
+        $ tree .
+        .
+        ├── big_file
+        └── folder
+            ├── file
+            └── other_file
+
+If you only want to synchronise `folder` and its content on your laptop the following will be automatically generated::
+
+        $ tree .
+        .
+        ├── big_file -> ../.data/big_file
+        └── folder
+            ├── file
+            └── other_file
+
+`big_file` is now a symbolic link and by mounting the remote directory you can still seemlessly access `big_file` through the network.

docs/source/index.rst

@@ -6,13 +6,26 @@
 Documentation for unisync
 =========================
 
-Unisync is a data synchronising tool built around `unison`_ 
+Unisync is a data synchronising tool built around `unison`_ and expending on it.
+
+Unisync tries to solve two problems that are often solved separately but never together :
+
+* Keeping your data synchronised between multiple machines (through a central server), examples of this are rsync and of course unison.
+* Being able to access and edit files stored on your server without having to download them, the gui interface of nextcloud for example.
+* And of course I want to be able to do all of this without ever having to leave my terminal.
+
+Unisync solves this problem by placing each file on your local machine but with only the selected files and folders being physically present on your drive,
+the others are replaced by symbolic links pointing to a directory that is mounted from your server.
+
+See this 
+:ref:`example_how_it_works`.
 
 .. _unison: https://github.com/bcpierce00/unison
 
 .. toctree::
    :maxdepth: 2
    :caption: Contents:
-       
+
+   example
    modules