Make minor improvements to the README.md file

[joker314]

This pull request makes two changes:

• a4b201e Give the top image some alternative text. The image adds value to the document by explaining that the repository concerns a website that details a way for websites to define a security policy. This information replaces the less descriptive previous alt attribute of "image"
• 5cf6259 Resolves a potential security-related issue. In particular, it resolves the security issue of reverse tabnabbing. In this particular case, the issue is not exploitable since GitHub ignores the request that the link open in a new tab; and markdown files do not usually open locally in the browser. To comply with best practices, we fix it anyway.