I don't know what documentation to include to show foreign-earned income. What if I don't have a W-2?

Part of my salary was paid by the French state last year (as maternity leave pay).  I don't have a w-2 for this income, but I do have a certificate from the French social security agency that lists the income I received.  Should I enclose this certificate with my tax return and explain the situation in a letter to the IRS?