Rename readme.txt to README.md & update links

[ashleydavies]

GitHub natively renders this file without any changes very nicely as a Markdown file, whereas it's currently rendering it in plaintext

See:
https://github.com/SFML/SFML.Net/blob/master/readme.txt
https://github.com/SFML/SFML.Net/blob/76f05c206da7f021f63797c28023ed59a2ec6215/README.md

[ashleydavies]

Also added a Travis badge to help identify where the CI is hosted at, although very happy to undo that if it's not a desired change!

[eXpl0it3r]

I believe it makes sense to align it with how the official SFML repo looks like. 👍

But since we're already using Markdown, might make sense to actually transform the URLs to text links.

[ashleydavies]

Thanks for the very prompt responses! Just updated the rest of the text links

[DemoXinMC]

All the changes seems fine, I would like to see the commits squashed, personally. Other than that, 👍

[eXpl0it3r]

Not really your fault, but the README hasn't been updated in a while, sooo a few smaller changes are desired 😅

[ashleydavies]

No worries! Think that's all fixed now

[ashleydavies]

Actually hold on this, I just realised there are links to this file within the repo so will fix those

[ashleydavies]

All good, there was just the one

[eXpl0it3r]

Ah just as I thought it was done, I saw some minor things again 🙈

[ashleydavies]

Just fixed those, will squash later when I get home from work

[eXpl0it3r]

Any chance to get it squashed? :)

[ashleydavies]

Sorry, lost track of time -- squashed now 😅