Document base package convention under Suggested Next Steps

[fctg-stevendoolan]

When using this template, engineers are likely to rename com.embabel.template to something similar, such as com.embabel.myproject, without realising that the com.embabel domain is reserved for code developed by Embabel Pty.

This was raised in embabel-agent#1557 by @igordayen:

@fctg-stevendoolan - please note, you are using domain "com/embabel", which is reserved for code developed by Embabel Pty.

It would be helpful to add a note about this convention under the Suggested Next Steps section of the README, so that engineers starting from this template know to choose an appropriate base package from the outset.