sync README.md with doc comments in elisp

The documentation in README.md was previously identical to that
in the Commentary section of use-package.el, modulo the following
differences:

  - No elisp comment ";; " prefix
  - Code blocks indented 4 columns not 2, as required by Markdown
  - Elisp symbols marked in backtick delimiters for monospace, not
    emacs backtick/forward tick pairs.

Unfortunately due to this duplication, sometimes only one of the two
files got updated, so they got out of sync.  This commit fixes that.

Fixes #46.
fix DRY violation by only having documentation in one place

The documentation in README.md was previously identical to that
in the Commentary section of use-package.el, modulo the following
differences:

  - No elisp comment ";; " prefix
  - Code blocks indented 4 columns not 2, as required by Markdown
  - Elisp symbols marked in backtick delimiters for monospace, not
    emacs backtick/forward tick pairs.

Unfortunately due to this duplication, sometimes only one of the
two files got updated, so they got out of sync.  With us all being
human, this is likely to continue to happen as long as the
duplication exists ;-)  Therefore since most users are likely to
encounter README.md before the elisp, and bearing in mind that
Markdown is a much more flexible format for documentation than
elisp comments (richer formatting, can be exported to numerous
other formats etc.), it is better to replace the docs in
use-package.el with a pointer to the README.md.