doc: add version in outputs document (#936)

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`version` is included in the outputs, and it would be helpful to use
instead of combine with major, minor, and patch.

See also:


https://github.com/google-github-actions/release-please-action/blob/cc61a07e2da466bebbc19b3a7dd01d6aecb20d1e/src/index.ts#L162-L193


https://github.com/googleapis/release-please/blob/72b0ab360c3d6635397e8b02f4d3f9f53932e23c/src/manifest.ts#L273-L280

Signed-off-by: Andy Zhang <37402126+AnzhiZhang@users.noreply.github.com>
Co-authored-by: Jeff Ching <chingor@google.com>

Author: AnzhiZhang

README.md

@@ -183,6 +183,7 @@ If you have a root component (path is `.` or unset), then the action will also o
 | `upload_url`         | Directly related to [**Create a release**](https://developer.github.com/v3/repos/releases/#response-4) API |
 | `html_url`           | Directly related to [**Create a release**](https://developer.github.com/v3/repos/releases/#response-4) API |
 | `tag_name`           | Directly related to [**Create a release**](https://developer.github.com/v3/repos/releases/#response-4) API |
+| `version`            | Number representing full semver value                                                                      |
 | `major`              | Number representing major semver value                                                                     |
 | `minor`              | Number representing minor semver value                                                                     |
 | `patch`              | Number representing patch semver value                                                                     |
@@ -201,6 +202,7 @@ This prefix allows you to distinguish values for different releases.
 | `<path>--upload_url`      | Directly related to [**Create a release**](https://developer.github.com/v3/repos/releases/#response-4) API |
 | `<path>--html_url`        | Directly related to [**Create a release**](https://developer.github.com/v3/repos/releases/#response-4) API |
 | `<path>--tag_name`        | Directly related to [**Create a release**](https://developer.github.com/v3/repos/releases/#response-4) API |
+| `<path>--version`         | Number representing full semver value                                                                      |
 | `<path>--major`           | Number representing major semver value                                                                     |
 | `<path>--minor`           | Number representing minor semver value                                                                     |
 | `<path>--patch`           | Number representing patch semver value                                                                     |