Code:

#!/usr/bin/env python3
import yaml

with open('file.yaml', 'r+') as stream:
  documents = yaml.load_all(stream, Loader=yaml.FullLoader)
  for data in documents:
    for k, v in data.items():
      if "kind" in k:
        if "Deployment" in v:
          data['appendhere'].update({'addedkey': {'alright', 'value'}})
  stream.seek(0)
  yaml.dump_all(documents, stream)
  # discards surplus content when new data is smaller than old data
  stream.truncate()

file.yaml

---
one_key: value
---
kind: Deployment
appendhere:
  subkey: subvaluekey
---
third_key: value


file.yaml after script has run

<empty>