fix missing documentation for testsuite option in test.json

stdout line matching supports count since commit
66d62a224e to fail if certain output is
present (count: 0) but it was never documented in the contribution
guidelines.

docs/markdown/Contributing.md

@@ -387,6 +387,7 @@ Each dict contains the following keys:
 
 - `line`
 - `match` (optional)
+- `count` (optional)
 
 Each item in the list is matched, in order, against the remaining
 actual stdout lines, after any previous matches. If the actual stdout
@@ -401,6 +402,10 @@ matched:
 | `literal` | Literal match (default) |
 | `re`      | regex match             |
 
+The `count` element determines how many times the line is expected, and allowed,
+to be in the output. If unspecified, it must appear "any number of times, but at
+least once".
+
 #### skip_on_env
 
 The `skip_on_env` key can be used to specify a list of environment variables. If