README: translate from reStructuredText to Markdown (#2197)

For #2196

**What type of PR is this?**

> Documentation

**What package or component does this PR mostly affect?**

README.rst

**What does this PR do? Why is it needed?**

Translates README.rst into README.md for more familiarity, easier
editing. I tried to keep this as direct as possible with minimal changes
to the text or formatting within text. I did fix punctuation and
formatting in a few places.

Markdown tables can't have multiline cells with internal formatting, so
I've translated the tables into paragraphs.

**Which issues(s) does this PR fix?**

For #2196

**Other notes for review**

There are a lot of documentation improvements needed. This is just a
first step: I don't want to mix content changes with language changes.

Author: jayconrod

BUILD.bazel

@@ -76,7 +76,7 @@ filegroup(
         "CONTRIBUTORS",
         "Design.rst",
         "LICENSE",
-        "README.rst",
+        "README.md",
         "WORKSPACE",
         "def.bzl",
         "deps.bzl",