Note src/negative_test/ in contrib doc (#1869)
Note src/negative_test/ in contrib doc (#1869)

The contrib doc describes how to add test cases for a schema well and
succinctly. However, as a new contributor, it was not clear that the
`negative_test` exists. This adds a note, keeping it a brief as
possible, to point contributors towards negative tests.
schemastore/schemastore,schemastore/schemastore