[fix/ack-headers] Add paragraph on top of Acknowledgements page

[felix-schwarz]

Description

• adds a paragraph on top of the Acknowledgements to provide additional context
• adds PLCrashReporter license to acknowledgements

Related Issue

https://github.com/owncloud/enterprise/issues/4284

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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