update README.md

Add the description of the import/export format

README.md

@@ -8,5 +8,19 @@ Before running the app, make sure to:
 
 To launch the app, run `__main__.py` using Python.
 
+## Import/export format
+The app uses a simple format for importing and exporting graph.
+It consists of a list of vertices of the graph.
+The syntax is as follows, with the values in square brackets being optional:
+
+`n1 [direction] n2 [w1] [w2]`, where
+- `n1` and `n2` are labels of the nodes that are connected
+- `[direction]` is only used in directed graphs, and could be either `->` (going from `n1` to `n2`), `<-` (going from `n2` to `n1`) and `<>` (going both ways)
+- `[w1]` is the weight of the vertex from `n1` to `n2`
+- `[w2]` is the weight of the vertex from `n2` to `n1`; is only used in the case of `n1 <> n2 w1 w2`
+
+Examples of valid graphs can be found in the `graph examples/` folder. 
+Note that as long as the file is _not binary_, the extension doesn't matter.
+
 ## Sample Images
 ![Sample Images](https://i.imgur.com/7GU4K6a.png)