docs: Note vulnerability disclosures

[matheusd]

This adds a note linking to fixed vulnerabilities disclosed after the upstream v0.11.0 release.

Close #124

[xaur]

Reads well.

As a small "nit" (if any), you may also use these links since they feel more "OG" compared to Google Groups:

• https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-October/002858.html
• https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-October/002857.html

Two other useful links are these emails from Antoine Riard who discovered the vulns

• https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-October/002859.html
• https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-October/002855.html

In the first link he talks about ecosystem implications of how tx relay standardness rules may be a risk for layer 2 protocols.

[matheusd]

Updated. Thank you for the original links!