Reorganized description of process for contributing to clarify/increase readability

Changed the keyword for emailing from 'contact' to 'email' per https://github.com/swcarpentry/styles/issues/178 and changed the description for how to use GitHub to reflect usage of the web interface.

Close #152.

Based on https://github.com/swcarpentry/lesson-example/pull/144/files

Clarified instructions for using GitHub to contribute, altered link for [issues] to direct to GitHub page on navigating issues to avoid FIXME issue with link when initializing, removed link to master repo to avoid FIXME issue with link when initializing. These suggestions were based on a pull request initially done elsewhere swcarpentry/shell-novice#639

Close #186

Close #178

Demo at https://github.com/swcarpentry/lesson-example/pull/132/.

This is based on https://github.com/swcarpentry/lesson-example/pull/127/.

Building `md` from `Rmd` in a script like this will break if the `methods` package is not called and the code includes class object routines. See https://github.com/r-spatial/sf/issues/478

See https://github.com/swcarpentry/workshop-template/issues/410

The consumer was removed by 079d7d50 (Updating contribution guidelines
in boilerplate, 2016-06-26).

1. Checking that `index.md` has `root` set to '.'
2. Checking that `_config.yml` has `root` set to '..'

An internally-defined Markdown link has the form [xxx][yyy].  If the
ID 'yyy' doesn't resolve, the text is left as-is, so we check for
that, then subtract those that reference configuration values using
'{{'.  It's a hack, but it'll catch at least a few things.