l10n: README: refactor to use GFM syntax

Format README.md using GFM (GitHub Flavored Markdown) syntax.

- In order to use more than 3 level headings, use ATX style headings
  instead of setext style headings.

- In order to add highlights for code blocks, use fenced code blocks
  instead of indented code blocks.

Signed-off-by: Jiang Xin <[email protected]>

Author: jiangxin