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.