Installation instruction update for more recent Debian and Ubuntu-bas…

…ed distributions (#147)

* Updated the installation documentation to promote pipx-based installation where applicable

* Fixed some markdown formatting

README.md

@@ -130,11 +130,27 @@ Specifically:
      instructions below may be more convenient on Windows.
 
 3. Finally, install ``paper2remarkable``:
-   ```
+   ```bash
    $ pip install paper2remarkable
    ```
    this installs the ``p2r`` command line program.
 
+> [!IMPORTANT]
+> On certain newer Debian and Ubuntu-based distributions, running 
+>
+> ```bash
+> $ pip install paper2remarkable
+> ```
+>
+> will raise an ```error: externally-managed-environment```.
+> Rather, run 
+>
+> ```bash
+> $ pipx install paper2remarkable
+> ```
+>
+> to install the  ``p2r`` command line program.
+
 **Optionally**, you can install:
 
 - [pdftoppm](https://linux.die.net/man/1/pdftoppm) (recommended for speed).