Refresh contribution guidelines.
Add upstream / username remotes.  Illustrate rebase.
Used  instead of deprecated function.
Merge pull request #512 from mkcor/doc_fix-deprecationwarning

Used feature.peak_local_max instead of deprecated function
Merge pull request #509 from stefanv/contributor_guidelines

DOC: Refresh contribution guidelines.
Merge branch 'master' of git://github.com/scikit-image/scikit-image