openforcefield/openforcefields: OpenFF-1.0.0 Release Candidate 2

Adds OpenFF-1.0.0 Release Candidate 2. This is taken from the QM-only refit performed in https://github.com/lpwgroup/forcebalance-qcarchive/releases/tag/v0.0.9. The new files, openff-1.0.0-RC2.offxml and openff_unconstrained-1.0.0-RC2.offxml, are copies of result.offxml in that release tarball, with tabs converted to spaces, author and date fields filled, and cosmetic attributes removed. The constraints in openff-1.0.0-RC2.offxml are copied from release candidate 1.