Update images in README to use relative links to images stored in repo, and specify widths

[cjcenizal]

By using relative links we can ensure they stay up-to-date if the images change. By specifying widths, we can ensure the images don't look too large.

Currently, links look like this:

<img style="max-width: 100%;" alt="Welcome to React-Search" src="https://raw.githubusercontent.com/vectara/react-search/main/images/projectLogo.png"/>

![React-Search input](https://raw.githubusercontent.com/vectara/react-search/main/images/searchInput.jpg)

But they can be this instead:

<img style="max-width: 100%;" alt="Welcome to React-Search" src="images/projectLogo.png"/>
<img src="images/searchInput.jpg" alt="React-Search input" width="800" />