docs(README): update screenshot display and add caption

Fixes #20

- Add figcaption to give credit to the font used in the screenshot
- Improve accessibility by adding an alt attribute to the image

README.md

@@ -2,7 +2,10 @@
 
 A terminal-based real-time satellite tracking and orbit prediction application.
 
-<p align="center"><img src="docs/screenshot.png" width=80%></p>
+<p align="center">
+    <img src="docs/screenshot.png" width="80%" alt="Screenshot"><br/>
+    <small><i>The font used in the screenshot is <a href="https://github.com/microsoft/cascadia-code">Cascadia Code NF</a>.</i></small>
+</p>
 
 ## Features