Documentation improvements

Great module, got it working but it needs to have better documentation, because setting it up can be a bit complex (especially if you don't work with commerce and rules every day).

Here is a suggestion of a more detailed installation process:

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The purchase field generated by this module allows you to choose a different 'action set' per product. This allows for maximum flexbility: different products can have different actions which you can customize through the rules module.

INSTALLATION
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If you need to evaluate/test this module, this would be the most simple setup, given a standard Drupal installation:

1. Download and activate the module and dependencies.
2. Also make sure you have enabled these commerce modules:
- Payment UI
- Order UI
- Payment Method Example
3. Create a field of type 'purchase' on the default article content type.
4. Create a Rules Component Action Set:
- Goto admin/config/workflow/rules/components and choose "Add new component"
- Choose "action set"
- In the next screen fill in all required labels/machine names and set 2 parameters: Data type "Node" and "User".
5. Create a node of type you used in step 3 and set a price. The action set should be the one you created.
6. Purchase the node.
7. You will find you have no balance. Order some balance through /balance.
8. Complete the order as admin/root (admin/commerce/orders) and use the payment example to simulate a payment.
9. You should have some balance and be able to purchase the node now.

Note 1: There is also a balance block you can place through the block configuration screen. It will always show your current balance.

Note 2: in above steps, the purchase field is attached to a node of a existing content-type, but you can use it on any entity. You will have to change the parameters on the action set rule accordingly.

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Furthermore, you mention "Number" as a required module. It is, but it's also a core module, maybe better to mention (core) behind it for clarity.