docs: README updated

| `sync_slow`  | A user is awaiting a synchronous response which needs to be returned before they can continue with their action, but which the user may accept a slower response | Displaying a full-text search response while displaying an amusement animation | 5s    |

| `async_fast` | An async process which may block a user from continuing with their user journey                                                                                  | MR diff update after git push                                                  | 15s   |

| `async_slow` | An async process which will not block a user and will not be immediately noticed as being slow                                                                   | Notification following an assignment                                           | 5m    |

| `custom`     | A user experience whose acceptable duration does not match the predefined urgency buckets                                                                        | Creating a merge request with an accepted threshold of 90s                     | Set by `apdex_threshold_s` |

**Custom Apdex thresholds**

Use `urgency: "custom"` when none of the predefined thresholds accurately represents the expected user experience. Custom urgency requires `apdex_threshold_s`.

```yaml

description: "Creating a new merge request in a project"

feature_category: "code_review_workflow"

urgency: "custom"

apdex_threshold_s: 90

```

`apdex_threshold_s` is measured in seconds, must be greater than `0`, and cannot exceed `600`.

Prefer the predefined urgency values unless the SLI would otherwise be permanently red or permanently green because the predefined thresholds do not match the experience.

## Usage