Update developer guide with the new diagrams

docs/DeveloperGuide.md

@@ -394,6 +394,10 @@ The below sequence diagram goes into more details on how the execution of the co
 1. `FavCommand` calls the static `FavCommand#createFavPerson(personToFav)` function which calls for the static `PersonUtil#createPersonWithFavouriteStatus(Person selectedPerson, Favourite favourite)` using the `selectedPerson` and a new `Favourite` with the value `true`.
 2. This static `PersonUtil#createPersonWithFavouriteStatus(Person selectedPerson, Favourite favourite)` function creates a new `Person` with the given `Favourite` and returns back to `FavCommand`.
 
+The following activity diagram summarizes what happens when a user executes a new `fav` command:
+
+![Fav Activity Diagram](images/FavActivityDiagram.png)
+
 Similarly, the following diagrams shows how the `unfav 1` command works:
 
 ![Unfav Command Sequence Diagram](images/UnfavSequenceDiagram.png)
@@ -418,6 +422,10 @@ The below sequence diagram goes into more details on how the execution of the co
 1. `UnfavCommand` calls the static `UnfavCommand#createUnfavPerson(personToUnfav)` function which calls for the static `PersonUtil#createPersonWithFavouriteStatus(Person selectedPerson, Favourite favourite)` using the `selectedPerson` and a new `Favourite` with the value `false`.
 2. This static `PersonUtil#createPersonWithFavouriteStatus(Person selectedPerson, Favourite favourite)` function creates a new `Person` with the given `Favourite` attribute and returns back to `UnfavCommand`.
 
+The following activity diagram summarizes what happens when a user executes a new `unfav` command:
+
+![Unfav Activity Diagram](images/UnfavActivityDiagram.png)
+
 ### \[Proposed\] Undo/redo feature
 
 #### Proposed Implementation