edit README.md; add column with sizes

[kraasch]

This is a modified version of, and should adhere to, the Contributing Guidelines.

• This PR contains housekeeping only (URL edits, copy changes etc) - if this is checked, delete other lines that don't apply

I read in the values of company sizes which people wrote in the company profiles and added those values into a column in the README.md.

I feel this can improve the usefulness for job seekers finding the right size companies for them, since the list is not that small.

Most company sizes fall in one of these patterns

• not provided (?)
• direct number provided (e.g. 300)
• range provided (e.g. 100-150)
• less than some number (e.g. <50)
• about some number (e.g. ~50)
• more than some number (e.g. 50+)

i.e. grep -vE '^(\?|[,0-9]+|[,0-9]+-[,0-9]+|<[,0-9]+|~[,0-9]+|[,0-9]+\+)$' catches them all.