Add a new AddLink() operation to Span. (#3678)

Fixes open-telemetry#454

Related to #3337

As the Messaging SIG merged its last OTEP 222, we will be adding operations
that require Links after Span creation, taking a direct route with `AddLink()`,
albeit without any of the new members suggested in #3337, e.g. `timestamp` (to be
discussed in a separate issue).

```
AddLink(spanContext, attributes /* optional */)
```

specification/common/README.md

@@ -123,7 +123,7 @@ at this time, as discussed in
 [data points](../metrics/data-model.md#metric-points),
 [Spans](../trace/api.md#set-attributes), Span
 [Events](../trace/api.md#add-events), Span
-[Links](../trace/api.md#specifying-links) and
+[Links](../trace/api.md#link) and
 [Log Records](../logs/data-model.md) may contain a collection of attributes. The
 keys in each such collection are unique, i.e. there MUST NOT exist more than one
 key-value pair with the same key. The enforcement of uniqueness may be performed