Added notes about copyright headers in code, and a reminder that (#146)

pull-requests ideally come with an explanation how the correctness of
the PR was established.

CODE_STYLE.md

@@ -48,6 +48,8 @@ Formatting
 * When modifying existing code that does not conform to this style, prefer
  matching the existing style
 
+* Files should have suitable copyright headers.
+
 Implementation
 --------------
 

CONTRIBUTING.md

@@ -20,3 +20,10 @@ Unnecessary code churn should be avoided unless as part of a pull request aimed
 at improving code quality, such as fixing repeated code style violations or
 renaming a function whose meaning is unclear.
 Such pull requests should not also introduce significant new functionality.
+
+It is desirable in a pull request to explain how the code presented
+has been verified and how the verification has been made
+reproducible. Ideally the pull request is accompanied by some form of
+automated verification that is presented in a way that the reviewers
+of the pull request can run. It is desirable that the pull request
+explains how it relates to the existing RISC-V architectural tests.