Update README.postgis (#1670)

Update README.postgis (#1670)

- Use consistent formatting in reStructuredText; see
  formatted result with rst2pdf or rst2html, or if you don't
  want to install docutils, you can copy/paste the text to
  http://www.tele3.cz/jbar/rest/rest.html
- Wrap lines to 80 columns, trim trailing whitespace, replace tabs
  with space chars
- Fix typos, and use consistent case and punctuation
- Alphabetize the directory structure
- Add ./libpgcommon to directory structure, and attempt to
  describe it
- Arrange style of REQUIREMENTS section a bit more consistent
  across components: removing repetition, stating what is required
  or optional, etc.
- When referencing PostGIS SQL functions, use the "ST_" prefix,
  where applicable
- Mention that both Raster and Topology extensions are built by
  default, add notes how build without them

Patch by Mike Toews

git-svn-id: http://svn.osgeo.org/postgis/trunk@9478 b70326c6-7e19-0410-871a-916f4a2858ee