Add Operation-Timeout header definition

SPEC.md

@@ -97,6 +97,11 @@ Headers that start with the `Nexus-Callback-` prefix are expected to be attached
 the handler. The callback request must strip away the `Nexus-Callback-` prefix. E.g if a Start Operation request
 includes a `Nexus-Callback-Token: some-token` header, the callback request would include a `Token: some-token` header.
 
+The `Operation-Timeout` header field can be added to inform the handler how long the caller is willing to wait for an
+operation to complete. This is distinct from the more general `Request-Timeout` header which is used to indicate the
+timeout for a single HTTP request. Format of this header value is number + unit, where unit can be `ms` for
+milliseconds, `s` for seconds, and `m` for minutes.
+
 The `Nexus-Link` header field can be added to associate resources with the start request. A handler may attach these
 links as metadata to underlying resources to provide end-to-end observabililty. See the [`Nexus-Link`](#nexus-link)
 section for more information.