Update CONTRIBUTORS with link to agreement

Signed-off-by: Richard Chapman <[email protected]>

CONTRIBUTORS

@@ -124,7 +124,7 @@ to fulfill the following requirements:
     should create a separate branch and pull request to fix them (or create an issue and
     let someone else fix them).
 4.  The code should pass code review by one of the maintainers (or someone they nominate
-    to review it for them.
+    to review it for them).
 5.  The code should compile, without warnings, on all supported targets.
 6.  If appropriate, new tests must accompany new functionality.
 7.  The commit messages should conform to the commit message guidelines described below
@@ -149,6 +149,10 @@ to address them without starting from scratch. We are fairly strict about most o
 above rules (and extremely strict about some of them) so it's not unusual for a new
 contributor to have a few pull-requests rejected the first time they are submitted.
 
+Note that before we can accept a change from you, we will need to have a signed copy of
+the contributory agreement on file (unless you are a LexisNexis employee) - see
+http://hpccsystems.com/sites/default/files/pdf/hpcc_contributor_agreement.pdf
+
 Commit messages
 ===============