Improve running_ringpop.md

[motiejus]

• Add note about broken table in RST
• change ring.change stat from gauge to counter
• fix documented location of tick-cluster
• add {membership,ring}.checksum-periodic to the stat table.

Related: uber/ringpop-go#156

[mennopruijssers]

It is possible to use inline html in markdown (https://daringfireball.net/projects/markdown/syntax#html).
Is that an option?
The other option is to convert this file to RST format and use the RST-table format right?

I think I would prefer one of those two options over a comment linking to the github page...