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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crates/nu-utils/standard_library/README.md

@@ -51,8 +51,12 @@ use /path/to/standard_library/std.nu
 > ```
 
 ## :pencil2: contribute to the standard library
+- all the commands of the standard_library are located in [`std.nu`](std.nu)
+- the tests are located in files that have a name starting with "test_", e.g. [`test_std.nu`](test_std.nu)
+- a test runner, at [`tests.nu`](tests.nu), allows to run all the tests automatically
+
 ### :wrench: add new commands
-- add new standard commands to [`std.nu`](std.nu), or preferably create a new submodule.
+- add new standard commands by appending to [`std.nu`](std.nu)
 - add associated tests to [`test_std.nu`](tests_std.nu) or preferably to `test_<submodule>.nu`.
     - define a new exported (!) `test_<feature>` command
     - import the `assert` functions you need at the top of the functions, e.g. `use std.nu "assert eq"`