readme: add mention that dot is required

Signed-off-by: Robin Jarry <rjarry@redhat.com>

README.md

@@ -9,6 +9,19 @@ export it in other formats.
 pip install --user sosviz
 ```
 
+System dependency to [`dot`](https://command-not-found.com/dot) from the
+[`graphviz`](https://graphviz.org/) package.
+
+```
+sudo dnf install graphviz
+```
+
+or:
+
+```
+sudo apt install graphviz
+```
+
 ## Usage
 
 ```
@@ -28,7 +41,7 @@ options:
  Output format (default: svg).
 ```
 
-## Examples
+## Example usage
 
 ```
 sosviz ~/tmp/sosreport > example.svg