Add note about using "[ci skip]" for doc updates, etc.

[karlcow]

When we are pushing commits, travis will repeat its dance for every single one, even when there is nothing to test, such as docs. etc.

There are two possibilities to manage this:

• ci skip

  Commits that have [ci skip] anywhere in the commit messages will be ignored. [ci skip] does not have to appear on the first line, so it is possible to use it without polluting your project's history.

• blacklist some branches
  We could imagine to have things like:

branches:
    except:
        - /^doc.*$/

and using always docIssueNumber for a branch which is about doc and other type of conventions.

[miketaylr]

If you push a branch with say, 10 commits it will only build for the last one. Seems useless for changes to the CHANGELOG, etc., but it doesn't really cost us anything.

But feel free to use ci skip in your commit messages. Good to know. Not sure what action there is for this bug.

[karlcow]

The action could have been the second option. Configure the travis config file if we are adopting names for branches OR to add a line in the doc to inform people about ci skip for the last commit in the branch. :)

[miketaylr]

I prefer the 2nd option, if someone would like to update docs. IMO having a branch naming convention seems like too much process for what we gain (and if you forget or do it incorrectly, there's not much point to it).

[miketaylr]

I went ahead and just pushed this to master.