fix: add documentation for wrapping steps

[d2lam]

Adding documentation for wrapping a step

Related: screwdriver-cd/screwdriver#616

[tkyi]

maybe:
before the template's install step
and
after the template's install step

And/or should we include an example of the combined screwdriver yaml?

jobs:
    main:
        image: node:6
        steps:
            - preinstall: echo pre-install
            - install: npm install
            - postinstall: echo post-install
            - test: npm test
            - echo: echo $FOO
        environment:
            FOO: bar
        secrets:
            - NPM_TOKEN

[d2lam]

I added template to make it more clear. I think the concept is simple enough that we wouldn't need to give a combined screwdriver yaml.

[tkyi]