Add std-rfc README (nushell#15066)

Copied the old README from `nu_scripts/stdlib-candidate/std-rfc` over to `nu-std/std-rfc` and
updated it with the latest info.

Author: NotTheDr01ds

crates/nu-std/std-rfc/README.md

@@ -0,0 +1,30 @@
+# `std-rfc`
+
+## Overview and Requirements
+
+This module includes potential candidate commands (and other definitions) for inclusion in the Standard Library (`std`) that is built in to Nushell. As a general guideline, candidates should:
+
+* Be general purpose features that will be useful to a number of users
+* Include doc comments for definitions and parameters that can be used with `help <command>`
+* Include tests
+* Since doc comments are fairly limited, additional documentation can be included
+  in a GitHub discussion. This documentation can then be moved to the main website when the feature
+  is promoted to `std`. See [this example](https://github.com/nushell/nushell/discussions/14935#discussion-7882769) for some `table` helpers.
+
+## Showcase and Discussion
+
+While primary feedback should take place in the PR, we have also established a [Drawing Board Discord Showcase](https://discord.gg/C3eMKV58MQ) which can be used for several purposes:
+
+* Ideation before a PR is submitted
+* Raise awareness of the feature
+* Short-term questions and discussion
+
+Note: The Drawing Board is not just for `std-rfc`. Please tag your topic with `std-library` if it is about a Standard Library idea.
+
+## Promotion Evaluation
+
+In general, new `std-rfc` features will be evaluated after they have been trialed for a sufficient period, which may vary depending on the feature. After that period, the feature may be:
+
+* Promoted to `std`
+* Removed from `std-rfc`
+* Or changes may be requested and then reevaluated later