Nightly Toolchain Note (#675)

## Type of change

<!-- (mark with an `X`) -->

```
- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other
```

## Objective

<!--Describe what the purpose of this PR is. For example: what bug
you're fixing or what new feature you're adding-->

Add a note about the required nightly toolchain. Without this people
will receive an error when attempting to run the `build-api.sh` script.

For posterity, the nightly toolchain is required for certain features
deemed unsafe. You can find more info here:
#512

## Code changes

<!--Explain the changes you've made to each file or major component.
This should help the reviewer understand your changes-->
<!--Also refer to any related changes or PRs in other repositories-->

- **README.md:** Add nightly install instructions

## Before you submit

- Please add **unit tests** where it makes sense to do so

README.md

@@ -86,6 +86,9 @@ This project uses customized templates which lives in the `support/openapi-templ
 These templates resolves some outstanding issues we've experienced with the rust generator. But we
 strive towards modifying the templates as little as possible to ease future upgrades.
 
+Note: If you don't have the nightly toolchain installed, the `build-api.sh` script will install it
+for you.
+
 ## Tests
 
 Many of the SDK tests are based on encrypted data provided by the other Bitwarden clients. In order

support/build-api.sh

@@ -22,5 +22,6 @@ npx openapi-generator-cli generate \
     -t ./support/openapi-template \
     --additional-properties=packageVersion=1.0.0
 
+rustup toolchain install nightly
 cargo +nightly fmt
 npm run prettier