Update README.md to describe Atree

[fxamacker]

Description

After first sentence about Atree, add:

It segments, encodes, and stores data into relatively small payloads.

This gives more context before going into technical detail.

Also, using "payloads" here matches terminology used in forum post about Atree by Jan.

UPDATE:

Based on comments/followups at https://github.com/onflow/atree/pull/332#pullrequestreview-1567739548 this is first paragraph:

Atree provides scalable arrays and scalable ordered maps. It segments, encodes, and stores data into relatively small, relatively fixed-size segments of bytes (aka payloads, registers, or slabs). This enables blockchains to only hash and transmit modified segments instead of the entire array, map, or large element.

Thanks @turbolent for feedback and suggestions :+1:

---

• [x] Targeted PR against main branch
• [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [x] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels