README: Upgrade to Markdown
This MR is almost a complement to !998 (closed) , since they both try to improve the appearance of the repository root, which is not essential but at least welcome.
GIMP has a beautiful site to the average user and a well-crafted site for the developer, both moderns. However, one small detail escapes in the repo: the README file seems to have been made and preserved since the 20th century. See:
Today, about 61% of the dependencies listed in the INSTALL file use markdown language to show a nice and useful front page on GitLab/GitHub (optionally see MD adoption list that I quickly annotated in a PDF: MD_adoption.pdf). The MD allows links to be accessed more easily, among other long-known advantages.
So, considering the cited numbers and the apparent benefits, this MR it's not just a matter of pure personal taste, it's a useful little trick to make a good impression on a potential new contributor when visiting the online repository, like this: