DOC: add a README to the standard library (#8304)

# Description
this PR a `README` to the standard library in
`crates/nu-utils/standard_library/` 👍

> **Note**
> you can have a look at what the `README` looks like
[here](https://github.com/amtoine/nushell/blob/doc/add-a-readme-to-the-standard-library/crates/nu-utils/standard_library/README.md#--welcome-to-the-standard-library-of-nushell--)

# User-Facing Changes
the user can now access some documentation about the standard library. 

# Tests + Formatting
```
$nothing
```

# After Submitting
```
$nothing
```

crates/nu-utils/standard_library/README.md

@@ -0,0 +1,67 @@
+<h1 align="center">
+  Welcome to the standard library of `nushell`!
+  <img src="https://media.giphy.com/media/hvRJCLFzcasrR4ia7z/giphy.gif" width="28"></img>
+</h1>
+
+The standard library is a pure-`nushell` collection of commands to allow anyone to build
+complex applications using standardized tools gathered incrementally.
+
+In this library, you might find `rust`-like `assert` commands to write tests, tools to
+manipulate paths and strings, etc, etc, ...
+
+## :toolbox: use the standard library in the REPL or in scripts
+in order to "import" the standard library to either the interactive [*REPL*][REPL] of
+`nushell` or inside some `.nu` script, you might want to use the
+[`use`](https://nushell.sh/commands/docs/use.html) command!
+```bash
+use /path/to/standard_library/std.nu
+```
+
+### :mag: a concrete example
+- my name is @amtoine and i use the `ghq` tool to manage `git` projects
+> **Note**  
+> `ghq` stores any repository inside `$env.GHQ_ROOT` under `<host>/<owner>/<repo>/`
+- the path to my local fork of `nushell` is then defined as
+```bash
+let-env NUSHELL_REPO = ($env.GHQ_ROOT | path join "github.com" "amtoine" "nushell")
+```
+- and the full path to the standard library is defined as
+```bash
+let-env STD_LIB = ($env.NUSHELL_REPO | path join "crates" "nu-utils" "standard_library")
+```
+> see the content of `$env.STD_LIB` :yum:
+> ```bash
+> >_ ls $env.STD_LIB | get name | str replace $env.STD_LIB "" | str trim -l -c "/"
+> ╭───┬───────────╮
+> │ 0 │ README.md │
+> │ 1 │ std.nu    │
+> │ 2 │ tests.nu  │
+> ╰───┴───────────╯
+> ```
+- finally we can `use` the standard library and have access to the commands it exposes :thumbsup:
+```bash
+>_ use std.nu
+>_ help std
+Module: std
+
+Exported commands:
+  assert (std assert), assert eq (std assert eq), assert ne (std assert ne), match (std match)
+
+This module does not export environment.
+```
+
+## :pencil2: contribute to the standard library
+### :wrench: add new commands
+- add new standard commands to [`std.nu`](std.nu)
+- add associated tests to [`tests.nu`](tests.nu)
+    - define a new `test_<feature>` before the `main`
+    - import the `assert` functions you need at the top of the functions, e.g. `use std.nu "assert eq"`
+    - add a call to `test_<feature>` at the bottom of the `main`
+
+### :test_tube: run the tests
+the following call should return nothing
+```bash
+nu ($env.STD_LIB | path join "tests.nu")
+```
+
+[REPL]: https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop