Add README.md file to extdata

[DimmestP]

From #85 Flic's code review

add some detail somewhere on what the inst/extdata folder (https://github.com/ewallace/tidyqpcr/tree/master/inst/extdata) is about, what it contains and when potential developers would need to get involved with this? I don't think this is a 'typical' R package structure item, so adding a bit of information about what's in there (a README within the folder?) and how it's used might help increase the 'bus factor' of the project!