README.md: Fix MD041 warnings

The following warnings are generated by using markdownlint analysis:
```
MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## To Build"]
```
Refer to markdown-lint [1] to fix MD041
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/041-first-line-h1.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I7d3d7108f64597c66e18f8ac54c7ddbd53817d8e
1 file changed