Antoine Stevan d1309a36b2
standard library: fix the readme (#8526)
# Description
as we now want to put all the library in `std.nu` alone, this PR removes
the mentions to "creating a separate submodule from `std.nu`" from the
`README` of the standard library and adds a few clarifications about the
structure of the library.

# User-Facing Changes
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```

# Tests + Formatting
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# After Submitting
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```
2023-03-20 17:05:49 +13:00
..
2023-03-17 21:29:15 +13:00