Prepare v5.3 release: CHANGELOG

[gsnw-sebast]

Preparation of CHANGELOG.md for the new release v5.3
The last two open pull requests are already included

@schweikert You only have to change the Next to fping 5.3 (<date>)

[coveralls]

coverage: 87.714%. remained the same
when pulling 698a92a on gsnw:prepare-v53_CHANGELOG
into 48e7e02 on schweikert:develop.

[schweikert]

Thank you Sebastian!