Improve README.md

[carrierpigeondev]

Small improvements to the README.md file including:

• Changes to the wording of "What is Fractal Bitcoin" section (see commits).
• Uppercased "Linux" in the "Run on Linux" header (from "Run on linux").
• Made the section on Docker have a clearer name (now "Run with Docker on Linux").
• Added requirement that git needs to be installed on the system (not present in minimal installs of some distros).
• Added requirement of Docker in the "Run with Docker on Linux" section as well as linked Docker documentation on how to install it on Linux
• Incorporated #4 into this PR as well as it is related to the README.md

Didn't add a "Minimum Requirements" section as it is not yet clear what the official requirements are. (See #2 )

[carrierpigeondev]

There's no commit messages or changelog, as far as I know, so I will close this commit.