Add style rule about implicit lambda captures.

style-guide.md

@@ -378,6 +378,12 @@ The KJ and Cap'n Proto libraries are designed to function correctly when compile
 
 We do NOT recommend disabling RTTI in your own code.
 
+### Lambdas
+
+Lamba capture lists must never use `=` to specify "capture all by value", because this makes it hard to review the capture list for possible lifetime issues.
+
+Capture lists *may* use `&` ("capture all by reference") but *only* in cases where it is known that the lambda will not outlive the current stack frame. In fact, they generally *should* use `&` in this case, to make clear that there are no lifetime issues to think about.
+
 ### Use of Standard libraries
 
 #### C++ Standard Library