Fix ctx.addIssue in transform to work correctly with parseAsync

[bentefay]

A recent PR added ctx.addIssue to transform: #1056

This is fantastic!

There appears to be a small bug - calling ctx.addIssue in transform does not work correctly when using parseAsync. This results in a confusing behaviour where parseAsync and parse return different results.

Reproduction

z
  .string()
  .transform((val, ctx) => {
    const parsed = parseInt(val);
    if (isNaN(parsed)) {
      ctx.addIssue({
        code: z.ZodIssueCode.custom,
        message: "Not a number",
      });
    }
    return parsed;
  })
  .parse("Not a number");

Expected: Zod error
Actual: Success

I've added a test to confirm it is now working correctly. I also took the chance to update the README to reflect this new feature.

[colinhacks]

Looks solid, and good catch!