README: expand description of packaging metadata

[scpeters]

This is not a typical pull request; it's just updating the README with an expanded description of the two types of data contained in this repository in a way that I think is more readable. Thanks in advance for helping to correct any inaccuracies in my phrasing.

[jlblancoc]

👍
I would prefer another wording instead of datasets, which for most of us in robotics has a different meaning than implied here... perhaps:

-The rosdistro repository stores two independent datasets used in ROS:
+This repository maintains:

[scpeters]

👍 I would prefer another wording instead of datasets, which for most of us in robotics has a different meaning than implied here

how about "This repository maintains two independent sets of packaging metadata"?

[christophebedard]

I also think that would be better.

[scpeters]

👍 I would prefer another wording instead of datasets, which for most of us in robotics has a different meaning than implied here

how about "This repository maintains two independent sets of packaging metadata"?

applied in 5170aea

[nuclearsandwich-ai]

Thanks!