add std lib link to README (#4938)

## Description
closes #4934 

## Checklist

- [ ] I have linked to any relevant issues.
- [ ] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] I have added tests that prove my fix is effective or that my
feature works.
- [ ] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [ ] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [ ] I have requested a review from the relevant team or maintainers.

Co-authored-by: Joshua Batty <[email protected]>

README.md

@@ -11,6 +11,8 @@ Sway is a language developed for the Fuel blockchain. It is heavily inspired by
 
 For user documentation, including installing release builds, see the Sway Book: <https://fuellabs.github.io/sway/latest/>.
 
+For Sway Standard library documentation, see: <https://fuellabs.github.io/sway/master/std/>
+
 Also view the technical reference for the Sway programming language: <https://fuellabs.github.io/sway/master/reference/>
 
 ## Building from Source