docs: add sfdx supported release type (#781)

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List `sfdx` as a supported release type
`sfdx` release type is included since release please
[v15.9.0](https://github.com/googleapis/release-please/releases/tag/v15.9.0),
which is used in the github action since
[v3.7.6](https://github.com/google-github-actions/release-please-action/releases/tag/v3.7.6).

Co-authored-by: Jeff Ching <[email protected]>

README.md

@@ -153,6 +153,7 @@ Release Please automates releases for the following flavors of repositories:
 | `php` | [A php composer package with composer.json and CHANGELOG.md](https://github.com/setnemo/asterisk-notation)
 | `ruby` | [A Ruby repository, with version.rb and CHANGELOG.md](https://github.com/google/google-id-token) |
 | `rust` | A Rust repository, with a Cargo.toml (either as a crate or workspace) and a CHANGELOG.md |
+| `sfdx` | A repository with a [sfdx-project.json](https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_ws_config.htm) and a CHANGELOG.md |
 | `simple` | [A repository with a version.txt and a CHANGELOG.md](https://github.com/googleapis/gapic-generator) |
 | `terraform-module` | [A terraform module, with a version in the README.md, and a CHANGELOG.md](https://github.com/terraform-google-modules/terraform-google-project-factory) |