README: fix markdown formatting
README: fix markdown formatting

- limit line length (does not affect rendering)
- add first-level header first
- converted google domain from hyperlinks into plain text
- add language specification for code fragments

Here is a command to run Markdown syntax check:

```
docker run --rm --network none -v "/$PWD:/markdown:ro" 06kellyjac/markdownlint-cli:0.21.0-alpine //markdown
```

All the changes above fixed the following list of errors:

```
/markdown/README.md:1 MD041/first-line-heading/first-line-h1 First line in file should be a top level heading [Context: "## wait-for-it"]
/markdown/README.md:3 MD013/line-length Line length [Expected: 80; Actual: 280]
/markdown/README.md:7 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
/markdown/README.md:21 MD013/line-length Line length [Expected: 80; Actual: 131]
/markdown/README.md:23 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
/markdown/README.md:30 MD013/line-length Line length [Expected: 80; Actual: 124]
/markdown/README.md:32 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
/markdown/README.md:39 MD013/line-length Line length [Expected: 80; Actual: 239]
/markdown/README.md:41 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
/markdown/README.md:48 MD013/line-length Line length [Expected: 80; Actual: 185]
/markdown/README.md:50 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
```

vishnubob/wait-for-it,vishnubob/wait-for-it