docs: add upgrading guide

UPGRADING.md

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+
+
+<!-- WIP, to be moved to google-cloud-python repository --->
+
+# Method Calls
+
+Methods now expect request objects. We provide a script that will convert most common cases automatically. 
+
+1. Install the library
+
+```py
+python3 -m pip install google-cloud-text-to-speech
+```
+
+2. `fixup_keywords.py` is shipped with the library. The script expects
+an input directory (with the code to convert) and an empty destination directory.
+
+```sh
+$ fixup_keywords.py --input-directory .samples/ --output-directory samples/
+```
+
+Before:
+```py
+from google.cloud import texttospeech
+
+client = texttospeech.TextToSpeechClient()
+
+voices = client.list_voices(language_code="no")
+```
+
+After:
+```py
+from google.cloud import texttospeech
+
+client = texttospeech.TextToSpeechClient()
+
+voices = client.list_voices(request={"language_code": "no"})
+```
+
+# Location of Types and Enums
+
+Types and Enums are available at the top level.
+
+Before:
+```py
+
+from google.cloud import texttospeech
+
+voice = texttospeech.types.VoiceSelectionParams(language_code="en-US")
+encoding = texttospeechAudioEncoding.MP3
+```
+
+
+After:
+```py
+from google.cloud import texttospeech
+
+voice = texttospeech.VoiceSelectionParams(language_code="en-US")
+encoding = texttospeech.AudioEncoding.MP3
+```