mention pip3 (#10)

It's probably an elementary thing but I couldn't install this application because my `$ python` was MacOS Python, which is v2.x and this application requires Python v3.x. Now, `pip` is v2.x Python application; to install using Python v.3.x you must call `pip3`.
motet-a · Aug 29, 2018 · dceab9f · dceab9f
1 parent cc3f0f5
commit dceab9f
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/README.md b/README.md
@@ -38,7 +38,7 @@ tags and indentation errors:
 
 ## Usage
 
-Install it with `pip install jinjalint`, then run it with:
+Install it with `pip install jinjalint` (or `pip3 install jinjalint` if you are using Python v.3.x), then run it with:
 
 ```sh
 $ jinjalint template-directory/