File size savings over avar1 release, as a README badge

[davelab6]

By my calculations...

File	glyphs	file-size	bytes per glyph
RobotoA2-avar2-VF.ttf	1,042	476,856	457.63
RobotoFlex[...].ttf	948	1,787,292	1,885.33

1,885.3291139241/457.6353166987 = 4.11x smaller
457.6353166987/1,885.3291139241 = 24.27349757%

I wonder if @sannorozco can quickly set up a https://shields.io badge to keep track of this number right on the README :)

[sannorozco]

Added at the beginning.

[davelab6]

Nice! This isn't quite what I meant, which is the badge should only have the percentage calculation result, eg 24%, which is "this" number, and the table isn't needed - although if you have a way to dynamically calculate it and reinsert/update the read me file on each commit, it's fine to keep it.

Maybe sheilds.io can't do the calculations and this is not easy to do though...

[davelab6]

@sannorozco says it seems like Sheild doesn't support calculation so this has to be done manually, which is fine, but then we can just have the markdown table and remember to update it periodically :) The current shield is display 'live' file size though.