Update README to reflect caveats and limitations of this gem #141
Description
The README should be updated to reflect the caveats and limitations of this gem.
In particular, we should clarify that this gem does not guarantee that mappings between Magento 1.x data and Shopify Transporter App formats will always work, because not all Magento 1.x data adheres to our default rules.
We should make it clear that even for Magento 1.x, there may be the need to apply custom mappings via custom pipeline stages.
This gem was created to provide some assistance in helping migrate Magento 1.x data, but it not intended to be used as a complete out-of-the-box solution. It should be clear to users that their mileage may vary, and should not completely rely on this gem to perform their data conversions.
It should also be made clearer that this gem is open source, and that it is currently supported by Shopify only on a best-effort basis.