Add knexfile.ts example to README.md

[jhuizingh]

This PR gives an example of how to do the knexfile.js in TypeScript. The JS version did not work for me, giving the below error. The modified example in the PR successfully worked.

Also, not directly related, I needed to run npm install -g knex before the migration generation command worked. Not sure if that would make sense to document somewhere in the readme too.

Error:

$ knex migrate:make create-users
(node:4651) UnhandledPromiseRejectionWarning: Error: Cannot find module './src/app'
Require stack:
- /home/jh/code/feathers-chat/knexfile.js
- /home/jh/.nvm/versions/node/v12.16.3/lib/node_modules/knex/bin/cli.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:957:15)
    at Function.Module._load (internal/modules/cjs/loader.js:840:27)
    at Module.require (internal/modules/cjs/loader.js:1019:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.<anonymous> (/home/jh/code/feathers-chat/knexfile.js:1:13)
    at Module._compile (internal/modules/cjs/loader.js:1133:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1153:10)
    at Module.load (internal/modules/cjs/loader.js:977:32)
    at Function.Module._load (internal/modules/cjs/loader.js:877:14)
    at Module.require (internal/modules/cjs/loader.js:1019:19)
(node:4651) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:4651) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

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