Update readme

Add 3.8.x release for Laravel 8.

Author: divine

README.md

@@ -68,6 +68,7 @@ Make sure you have the MongoDB PHP driver installed. You can find installation i
  5.8.x    | 3.5.x
  6.x      | 3.6.x
  7.x      | 3.7.x
+ 8.x      | 3.8.x
 
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