doc: Clarify wording about testing in README.md

Weaken and clarify the wording a bit, it currently implies that we get
more pull requests than we can ever handle which discourages
contribution.

README.md

@@ -49,8 +49,8 @@ Testing
 -------
 
 Testing and code review is the bottleneck for development; we get more pull
-requests than we can review and test. Please be patient and help out, and
-remember this is a security-critical project where any mistake might cost people
+requests than we can review and test on short notice. Please be patient and help out by testing
+other people's pull requests, and remember this is a security-critical project where any mistake might cost people
 lots of money.
 
 ### Automated Testing