+ section pipx inject (pypa#1170)

Co-authored-by: Bernát Gábor <bgabor8@bloomberg.net>

README.md

@@ -225,6 +225,14 @@ The pip syntax with `egg` must be used when installing extras:
 pipx install "git+https://github.com/psf/black.git#egg=black[jupyter]"
 ```
 
+### Inject a package
+
+If an application installed by pipx requires additional packages, you can add them with pipx inject. For example, if you have ```ipython``` installed and want to add the ```matplotlib``` package to it, you would use:
+
+```
+pipx inject ipython matplotlib
+```
+
 ### Walkthrough: Running an Application in a Temporary Virtual Environment
 
 This is an alternative to `pipx install`.