chore: Add back in the output formats

Agreed with @kzantow - the output formats is essential and doesn't bulk the readme out too much to have it in here.

README.md

@@ -95,6 +95,27 @@ The above output includes only software that is visible in the container (i.e.,
 syft <image> --scope all-layers
 ```
 
+### Output formats
+
+The output format for Syft is configurable as well using the `-o` (or `--output`) option:
+
+```
+syft <image> -o <format>
+```
+
+Where the `formats` available are:
+- `syft-json`: Use this to get as much information out of Syft as possible!
+- `syft-text`: A row-oriented, human-and-machine-friendly output.
+- `cyclonedx-xml`: A XML report conforming to the [CycloneDX 1.4 specification](https://cyclonedx.org/specification/overview/).
+- `cyclonedx-json`: A JSON report conforming to the [CycloneDX 1.4 specification](https://cyclonedx.org/specification/overview/).
+- `spdx-tag-value`: A tag-value formatted report conforming to the [SPDX 2.3 specification](https://spdx.github.io/spdx-spec/v2.3/).
+- `[email protected]`: A tag-value formatted report conforming to the [SPDX 2.2 specification](https://spdx.github.io/spdx-spec/v2.2.2/).
+- `spdx-json`: A JSON report conforming to the [SPDX 2.3 JSON Schema](https://github.com/spdx/spdx-spec/blob/v2.3/schemas/spdx-schema.json).
+- `[email protected]`: A JSON report conforming to the [SPDX 2.2 JSON Schema](https://github.com/spdx/spdx-spec/blob/v2.2/schemas/spdx-schema.json).
+- `github-json`: A JSON report conforming to GitHub's dependency snapshot format.
+- `syft-table`: A columnar summary (default).
+- `template`: Lets the user specify the output format. See ["Using templates"](#using-templates) below.
+
 ### Supported Ecosystems
 
 - Alpine (apk)