Add missing documentation for negative floats, as introduced in PR#127

[technimad]

In PR #127 support for negative floats was introduced, as a fix for issue #126 .

Documentation added in that PR is not present anymore, this PR brings all docs up to date. In the docs the mention that floats must be greater than 0 is removed.

Functionality is still present:

• test exists: https://github.com/splunk/eventgen/blob/develop/tests/large/test_token_replacement.py#L56
• main regex: https://github.com/splunk/eventgen/blob/develop/splunk_eventgen/lib/eventgentoken.py#L179