Update compatibility.md

Author: IslandRhythms

docs/compatibility.md

@@ -18,6 +18,7 @@ Below are the [semver](http://semver.org/) ranges representing which versions of
 
 | MongoDB Server |           Mongoose            |
 | :------------: | :---------------------------: |
+|     `7.x`      |           `^7.4.0`            |
 |     `6.x`      |      `^6.5.0 \| ^7.0.0`       |
 |     `5.x`      |      `^6.0.0 \| ^7.0.0`       |
 |    `4.4.x`     | `^5.10.0 \| ^6.0.0 \| ^7.0.0` |