Add breaking markdown table change to release notes.

Parsedown supports GitHub Flavoured Markdown tables, e.g.:

|A title|B title|
|-------|-------|
|A cell |B cell |

For less typing:

|A title|B title|
|-|-|
|A cell |B cell |

Parsedown 1.7.4 allowed even less typing:

|A title|B title|
|||
|A cell |B cell |

Even:

|A title|B title|
|
|A cell |B cell |

See the online demo (which isn't Parsedown 1.8.0 at the time of posting). However, the latter two are not supported in Parsedown 1.8.0. This should be listed as a breaking change in the current 1.8.0 release notes.