Improve README image alt text for accessibility (#5058)

Author: aryan1752

README.md

@@ -30,9 +30,9 @@ is audio-visual; it produces graphics, artwork and music. Here are a
 couple of screenshots to give you an idea of how the application looks
 like:
 
-![alt tag](./screenshots/Screenshot-1.png)
+![Music Blocks workspace showing block-based music programming](./screenshots/Screenshot-1.png)
 
-![alt tag](./screenshots/Screenshot-2.png)
+![Music Blocks rhythm and melody blocks interface](./screenshots/Screenshot-2.png)
 
 Visit the Music Blocks website for a hands on experience:
 [https://musicblocks.sugarlabs.org](https://musicblocks.sugarlabs.org).