README.md: Link to the latest manual

(cherry picked from commit 455611b0b4e97290a858687142805babc9929673)

Created by  Eelco Dolstra  on March 13, 2020
T5JJSEWKPTJOFUPVPXUFGYX2GP362HVUNJ4HENDSBV5CGWQ7HG3QC
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