Explain why pipx, but allow pip too

As suggested by Lecrapouille and cgestes
in your-tools#332, your-tools#366, your-tools#343.

Author: gdubicki

README.md

@@ -19,11 +19,17 @@ It can be [seen in action on asciinema.org](https://asciinema.org/a/131625).
 
 ## Installation
 
-The recommended way to install `tsrc` is to use [pipx](https://pipxproject.github.io/pipx/).
+Make sure to have Python **3.7** or later installed.
 
-* Make sure to have Python **3.7** or later installed.
-* Install `pipx`
-* Run `pipx install tsrc`.
+The **recommended** way to install `tsrc` is to use [pipx](https://pipxproject.github.io/pipx/), because it creates an isolated environment for the app:
+```shell
+pipx install tsrc
+```
+
+If you know what you are doing, you can also use plain `pip`:
+```shell
+pip install tsrc
+```
 
 ## Usage Example