Update the installation instructions in the README.
Tried my hands on a more thorough revision of the installation instructions in the README. See my comment !535 (comment 8231) for further details. The revised document can be read here (the diffs look pretty garbled): https://git.purrdata.net/jwilkes/purr-data/-/blob/5781aa0f9d0b53b54cec4800101c0685ae86b388/README.md