Added std-rfc README with installation instructions (#1030)

Includes a `README.md` for the `std-rfc` modules with 
information on how to checkout *just* `std-rfc` from the repo.

stdlib-candidate/std-rfc/README.md

@@ -0,0 +1,33 @@
+`std-rfc` is the root directory for modules and commands under consideration for the Nushell [Standard Library](https://www.nushell.sh/book/standard_library.html).
+
+As a part of the the larger nu_scripts mono-repo, users may find it more convenient (and efficient) to checkout `std-rfc` separately.
+
+This can be done in Nushell using:
+
+```nushell
+let spec = {
+  repo: 'https://github.com/nushell/nu_scripts'
+  sparse_dir: 'stdlib-candidate/std-rfc'
+  branch: 'main'
+}
+
+# Change to the parent directory location before running
+let install_dir = './nu_scripts'
+
+mkdir $install_dir
+git -C $install_dir init
+git -C $install_dir remote add origin $spec.repo
+git -C $install_dir sparse-checkout set --no-cone --sparse-index $spec.sparse_dir
+git -C $install_dir fetch --depth 1 --filter=blob:none origin $spec.branch
+git -C $install_dir checkout $spec.branch
+```
+
+Then imported using:
+
+```nushell
+use ./nu_scripts/stdlib-candidate/std-rfc/<module>
+# or
+use ./nu_scripts/stdlib-candidate/std-rfc/<module> *
+```
+
+A sparse git checkout like the one above requires the full parent path from the repository, but you can symlink the `std-rfc` directory to a more convenient location if you prefer.