chore: simplify PR description template

GitHub will automatically prepend the description based on the commit message so the text that reads "please put a message here" is redundant. Also, move the link to the contributing guide to the end.

Author: Elad Ben-Israel

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,11 +1,10 @@
-<!-- Please write here a description of what is included in your pull request -->
+
 
 ----
 
-<!-- 
-Please read the [contribution guidelines][1] and follow the pull-request checklist.
+*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*
 
-[1]: https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md
+<!-- 
+Please read the contribution guidelines and follow the pull-request checklist:
+https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md
  -->
-
-*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*