Update continuous and categorical sections of table in README

[Steph-Chong]

Fixes #647 - using @nicholasloveday's suggestions

Reduces the length of the continuous and categorical sections.

I think it looked okay in GitHub on my fork and looked okay in readthedocs when I built it locally.

[Steph-Chong]

I have now udpated this PR following offline feedback from Tennessee. I also took the opportunity to make one minor edit in each of the three paragraphs following the table.

My remaining questions are:

1. Should the order of the two sentences immediately above the table be swapped? E.g., something along the lines of: "Click here for the full list of metrics, tools and statistical tests included in scores. Below is a curated selection:"

2. Something about "17 binary contingency table (confusion matrix) metrics" looks slightly odd to me (but that might just be me). A part of me would like to instead say "17 x binary contingency table (confusion matrix) metrics" - but I am not sure if that is worse and/or potentially confusing?

[tennlee]

I have pushed this to 228, so it should render on readthedocs shortly

[tennlee]

https://scores.readthedocs.io/en/228-documentation-testing-branch/

Ping @nicholasloveday what do you think?

[nicholasloveday]

Looks good to me. Thanks for this update.