Add breaking change note - string attributes no longer implicitly converted

[IMBurbank]

Add breaking change note to v2.0.0 - string attributes no longer implicitly converted.

Summary:

Add breaking change note that non-string properties will no longer be converted when saved to a string attribute.

For example, a mongoose ID object will no longer be automatically converted and saved as a string.

GitHub linked issue:

Closes #870

#---

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement

• Test added to report bug (GitHub issue #--- @---)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Are all the tests currently passing on this PR? (select 1):

• Yes
• No

Other:

• I have read through and followed the Contributing Guidelines
• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have ensured the following commands are successful from the root of the project directory
  • npm test
  • npm run lint
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have filled out all fields above

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