Add loop-contracts doc to SUMMARY (#3886)

Add documentation of loop contracts to `SUMMARY.md` so that it will show
up in the book.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Author: qinheping

docs/src/SUMMARY.md

@@ -22,6 +22,7 @@
     - [Coverage](./reference/experimental/coverage.md)
     - [Stubbing](./reference/experimental/stubbing.md)
     - [Contracts](./reference/experimental/contracts.md)
+    - [Loop Contracts](./reference/experimental/loop-contracts.md)
     - [Concrete Playback](./reference/experimental/concrete-playback.md)
 - [Application](./application.md)
   - [Comparison with other tools](./tool-comparison.md)