Document support for mutations in GraphQL

[keharper]

Type of Doc:

• New topic
• Topic Update, Devdoc link:
• Other

Magento 2 GitHub Issue: magento/graphql-ce#74

Magento 2 GitHub PR: magento/graphql-ce#114

Magento 2 version:

• 2.1.X
• 2.2.X
• 2.3.X
• back port
• forward port

Notifications of Changes:

• Changes to Admin interface?
• New software requirements?
• New REST endpoints or updated GraphQL query syntax?
• New command line option?

Content:

Create a new topic about mutations

[shrielenee]

@keharper Is this valid for Squashtoberfest? If so, let's add that Project tag!

[keharper]

Completed with #3825