README.md: Provide `alt` text description for imgs

Add the `alt` attribute in order to describe the contents of an image
file. This is important for several reasons:

* Provides search engines the useful information about the subject
  matter of the image, allowing them to use this information to help
  determine the best image to return for a user's query.

* Helps users that may not be able to see images on web pages better
  understand the content (e.g.:people with visual impairments, people
  using screen readers or that have low-bandwidth connections).

Ref: https://support.google.com/webmasters/answer/114016?hl=en