docs(gatsby-image): Add SEO advice to the readme (#15056)

* Add new advice on the "stuff to be aware" section

During my report on github I have been asked to point out this issue on the documentation. 
You can find more details and the discussion here: #13742

* Update packages/gatsby-image/README.md

Approve suggestion

Co-Authored-By: Ward Peeters <ward@coding-tech.com>

* fix lint

Author: anonimoconiglio

packages/gatsby-image/README.md

@@ -421,6 +421,7 @@ While you could achieve a similar effect with plain CSS media queries, `gatsby-i
 
 - If you want to set `display: none;` on a component using a `fixed` prop,
   you need to also pass in to the style prop `{ display: 'inherit' }`.
+- Be aware that from a SEO perspective it is advisable not to change the image parameters lightheartedly once the website has been published. Every time you change properties within _fluid_ or _fixed_ (like _quality_ or _maxWidth_), the absolute path of the image changes. These properties generate the hash we use in our absolute path. This happens even if the image didn't change its name. As a result, the image could appear on the image SERP as "new" one. (more details [can be found on this issue](https://github.com/gatsbyjs/gatsby/issues/13742))
 - By default, images don't load until JavaScript is loaded. Gatsby's automatic code
   splitting generally makes this fine but if images seem slow coming in on a
   page, check how much JavaScript is being loaded there.