Update xblocks-contrib readme

[farhan]

Update the readme explaining how to do the development in xblocks-contrib

Example steps:

1. tutor mount xblocks-contrib repo
2. run npm run build

Need:

• Recently MIT team developers shared the issue of Video Block rendering issue while doing development in xblocks-contrib, reason was developer missed npm run build command to prepare the public assets.
• They also requested for the watcher for the continous development

[farhan]

PR: #233