Better document chunk size limit

With current readme, users might think we always truncate words.
The purpose of this lib is the exact opposite.

The word truncating is an edge case where there would be no space at all in the limit of the chunk size.

Let's make that clearer in the readme!

Start with the README and review the section describing chunk size limits and word truncation. Clarify that words are only truncated as an edge case when no space fits within the limit, then verify that the documentation accurately explains the library's purpose.

Written by the indexing model from the issue text.