Warn about passing non-unicode data to keywords in README

Many users don't know that to pass data such as integers into keywords, you must use the syntax (for example) `${5}` instead of `5`. I should mention this explicitly in the readme.

Start with the repository README, which is the only file identified by the issue. Add an explicit explanation of how to pass non-Unicode values such as integers to keywords, including the stated syntax example, and verify that the guidance is clear to users.

Written by the indexing model from the issue text.