add gov.uk & skillsmap

[dmorgantini]

• Add GOV.UK as they have published a very comprehensive (if pretty generic) set of career ladders
• Add SkillsMap.io, the first company focused on adding structure around engineering career ladders
• Remove ChartBeats as they no longer publish their engineering ladders.

[benjmin-r]

Thanks a lot David, for your PR and the work you put in.
However, I'd prefer if the skillsmap entry was split in two, to present the Tes ladder with your video as one, and then create a separate section for the skillsmap tool. I'll probably do this next week or if you find the time and would like to do it before that, you're of course more than welcome to 😃
Also, if you make another change, be sure to add your name to the contributors section of the post. Except if you don't want your name there, which of course would also be fine.

[dmorgantini]

Certainly happy to do so. Will add a commit today.

[benjmin-r]

Thanks a lot again, David, for your work in extending the ladder list.
I restructured the Tes entry slightly and got rid of the last paragraph of the Skillsmap entry, as it didn't fit the style of the other entries, because it describes the offer of a company rather than a ladder (or the tool surrounding ladders).