[JOSS-review] statement of need in readme.md

[strengejacke]

This one is related to openjournals/joss-reviews#978

While there's a clear statement of need in the paper, the background section of the readme.md could elaborate a bit more on why tidyLPA is needed, also in the context of what already exists. It's just a minor issue, I would say adding a small sentence to the background should be OK.

[jrosen48]

Thank you for this helpful issue! I added the following to the second paragraph of the Background section of the README:

The package is designed and documented to be easy to use, especially for beginners to LPA, but with fine-grained options available for estimating models and evaluating specific output as part of more complex analyses.