Add troubleshooting section to README

[akheron]

Type errors

Type errors like these are hard to decipher, add explanation:

// index.ts:24:7 - error TS2322: Type 'Route<ExpressContext, Response<400, string, undefined> | Response<200, string, undefined>>' is not assignable to type 'Route<ExpressContext, Response<400, string, undefined> | Response<200, "OK", undefined>>'.
//   Type 'Response<400, string, undefined> | Response<200, string, undefined>' is not assignable to type 'Response<400, string, undefined> | Response<200, "OK", undefined>'.
//     Type 'Response<200, string, undefined>' is not assignable to type 'Response<400, string, undefined> | Response<200, "OK", undefined>'.
//       Type 'Response<200, string, undefined>' is not assignable to type 'Response<400, string, undefined>'.
//         Type '200' is not assignable to type '400'.

The real problem usually is that the type of body doesn't match with the annotated response types.

Express compatibility

In express, res.send(200) writes OK and sets Content-Type: text/plain; charset=utf-8. Typera doesn't.