README and vignette tweaks

[maelle]

• Fix #196
• Moved parts of the README to R Markdown fragments (in man/rmdhunks) in order to be able to re-use them from the vignette
• Added said fragments to the vignette
• Added a short pitch at the beginning of the README (I think it's important to underline how easy getting started can be)
• Moved the "how it works in a lot of details" to a dedicated vignette to de-clutter the README (I am afraid a too long README might look daunting?)

[codecov-io]

Codecov Report

Merging #199 (2ceed20) into master (27db7bf) will increase coverage by 0.62%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   79.98%   80.61%   +0.62%     
==========================================
  Files          39       40       +1     
  Lines        1574     1733     +159     
==========================================
+ Hits         1259     1397     +138     
- Misses        315      336      +21     

Impacted Files	Coverage Δ
R/skip-vcr-off.R	50.00% <0.00%> (ø)
R/insert_cassette.R	93.54% <0.00%> (+0.44%)	:arrow_up:
R/cassette_class.R	85.98% <0.00%> (+1.01%)	:arrow_up:
R/response_class.R	77.77% <0.00%> (+2.16%)	:arrow_up:

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[maelle]

I made the changes but I would like to keep the YAML in the README.Rmd because it allows me to use the RStudio knit button to knit the README. :smile_cat: Only if it doesn't break your Make workflow, of course.

[sckott]

we can leave the readme yaml. knitr doesnt know to pull that yaml out of the .md i guess, probably ok though