(WIP) adams description

[yizhang-yiz]

Submission Checklist

• Builds locally
• Declare copyright holder and open-source license: see below

Summary

This PR partially answers @bbbales2 's call in #333 to improve ODE documentation. It provides a short description of Adams-Moulton integrator and its intended usage.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Metrum Research Group

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bbbales2]

Good