Add guidance in README on how to set span attributes

Accessing span attributes is most likely accessed behind a lock. To avoid locking and unlocking continuously it is preferable span attributes are built up separate from the span and then set once on the span.

Eg replace
```swift
span.attributes["attr1"] = value1
span.attributes["attr2"] = value2
...
```

with

```swift
var attributes = span.attributes
attributes["attr1"] = value1
attributes["attr2"] = value2
...
span.attributes = attributes
```

Start in the README and review the existing guidance around span attributes. Add the issue's example showing attributes being built separately and assigned once to avoid repeated locking, then verify the README clearly explains when to use this pattern.

Written by the indexing model from the issue text.