DOC: add links to other pages to contributing.rst
Merge pull request #6227 from y-p/PR_travis3

BLD: fix travis speedup
TST: add tests for algos.factorize (GH6212)
Merge pull request #6231 from jreback/test_factorize

TST: add tests for algos.factorize (GH6212)
Merge pull request #6207 from jorisvandenbossche/doc-contributing-update

DOC: add links to other pages to contributing.rst
BLD: more @network message skipping
BLD: fix travis caching on py3
BLD/DOC: add doc building to travis job. for free