Add 'Create' step to the plop master/detail template. And add it to the getting started guide

[mostafaelganainy]

In the plop template that creates a master/detail module. Theres is a missing step to create a new post.

It's required to do the following :

1. Create a template that creates a form using RelayForm to create a Posts form
2. The template has to be generic just like the other files in the "master/details" plop template

You can refer to the following file as an example for how to use RelayForm and how to consume mutations.

Sample Relay Form Usage

Also after finishing this, the project's ReadME, should be updated to reflect the mentioned change.