Add detailed docs on contributing upstream

[ethanwhite]

See #619 and #620 for motivation and draft of language/approach.

[atyre2]

After a second try, much tears and hair pulling, I think the following would be helpful

first check that upstream repository is linked:

git remote -v

if not

git remote add upstream https://github.com/datacarpentry/semester-biology.git
git fetch upstream

make changes for class

git add stuff.md
git commit -m "Changes for just your class"
<make changes>
git add stuff.md
git commit -m "Upstream changes"
git log
<note hash(es) for upstream changes commit(s)>
git checkout upstream/gh-pages
git checkout -b upstream-changes
git cherry-pick firsthash secondhash
git push origin upstream-changes
<open PR>
<reattach your head!>
git checkout gh-pages