Merge pull request #357 from benjeffery/readme_tweak

Remove readthedocs status from readme

README.md

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 ``tsdate`` is a scalable method for estimating the age of ancestral nodes in a 
 [tree sequence](https://tskit.dev/tutorials/what_is.html). The method uses a coalescent prior and updates node times on the basis of the number of mutations along each edge of the tree sequence (i.e. using the "molecular clock").