[WCP]alt attribute for img tag

[lyndaidaii]

Summary of the changes (in less than 80 characters):

use reg to add alt = image to fix accessibility.

This is short term quick fix. we need come up a better long-term solution.
For now, image just temporary image description.

Addresses https://github.com/NuGet/Engineering/issues/4784
Issue created here to track for long term solution: #9405

[joelverhagen]

Small comments

[joelverhagen]

Please track an issue (NuGet/NuGetGallery is fine) so we can take a better approach. Ideally package authors know about this problem and can remediate their own READMEs so they are more accessible. We can maybe block, maybe warn. Design required :)