message: docs: add missed handler README
diff: @@ -416,6 +416,7 @@ The fourth cache behaviour handles next API requests `api/*`.
  | runtime | `string\|object` | `nodejs12.x` | When assigned a value, both the default and api lambdas will be assigned the runtime defined in the value. When assigned to an object, values for the default and api lambdas can be separately defined | |
  | memory | `number\|object` | `512` | When assigned a number, both the default and api lambdas will be assigned memory of that value. When assigned to an object, values for the default and api lambdas can be separately defined | |
  | timeout | `number\|object` | `10` | Same as above |
  +| handler | `string` | `index.handler` | When assigned a value, overrides the default function handler to allow for configuration. Copies `handler.js` in route into the Lambda folders. Your handler MUST still call the `default-handler` afterwards or your function won't work with Next.JS |
  | name | `string\|object` | / | When assigned a string, both the default and api lambdas will assigned name of that value. When assigned to an object, values for the default and api lambdas can be separately defined |
  | build | `boolean\|object` | `true` | When true builds and deploys app, when false assume the app has been built and uses the `.next` `.serverless_nextjs` directories in `nextConfigDir` to deploy. If an object is passed `build` allows for overriding what script gets called and with what arguments. |
  | build.cmd | `string` | `node_modules/.bin/next` | Build command |
  