Changes to be made in README

These are the changes to be made to the README as described in rOpenSci onboarding.

[x] Introduce airquality at first usage, not after second plot (https://github.com/njtierney/visdat/commit/1985f6d1d01d3e801a098790ee23e55efd7a76c4)
[x] Fix wording around <0.1% missingness: "When there is <0.1% of missingness, vis_miss indicates that there is >1% missingness." (https://github.com/njtierney/visdat/commit/1985f6d1d01d3e801a098790ee23e55efd7a76c4)
[x] Link to the vignettes from the README. (https://github.com/njtierney/visdat/commit/1985f6d1d01d3e801a098790ee23e55efd7a76c4) (https://github.com/njtierney/visdat/commit/822cc87ef2249cca3be9ae8d760139ef66a645b3)
[x] Link "experimental" parts of visdat README to "experimental" vignette (https://github.com/njtierney/visdat/commit/1985f6d1d01d3e801a098790ee23e55efd7a76c4)
[x] Demonstrate differences between vis_dat and vis_guess, by putting them side by side by adding chunk options: fig.show='hold', out.width='50%'. (https://github.com/njtierney/visdat/commit/1985f6d1d01d3e801a098790ee23e55efd7a76c4)
[x] Elaborate on what vis_dat_ly is doing. (https://github.com/njtierney/visdat/commit/385b52a104375bf70cb516a7c71d459d6ce13a6a)
[x] Construct a small example of what might happen when visualising expectations. (https://github.com/njtierney/visdat/commit/445b974538099638aa7831074d99591b5de3db30)
[x] Add guidelines for contribution in README, and CONTRIBUTION.md (https://github.com/njtierney/visdat/commit/c7ea5bb6bb12a43719fc4dbdfbccc22153072b62)
[x] Make sure that the excessive warnings messages are fixed

ropensci / visdat