add research notice to readme

[yochannah]

... so contributors can be made aware this repo is participating in a study for the next year :)

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[mcguinlu]

Thanks for this @yochannah! Have also added it to the README.Rmd (which is rendered into the .Md file), and will merge in once checks pass.

Just pinging common collaborators (@AJFOWLER @rdboyes) so that you are aware any future contributions will form part of the study dataset. Full note is reporduced below for reference:

### Research notice

Please note that this repository is participating in a study into sustainability of open source projects. Data will be gathered about this repository for approximately the next 12 months, starting from June 2021.

Data collected will include number of contributors, number of PRs, time taken to close/merge these PRs, and issues closed.

For more information, please visit [the informational page](https://sustainable-open-science-and-software.github.io/) or download the [participant information sheet](https://sustainable-open-science-and-software.github.io/assets/PIS_sustainable_software.pdf).