Update contributor guidelines for authoring examples

[duncandewhurst]

• Closes Update contributor guidlines for authoring examples #286

I figure the guidelines are best illustrated with an example, which I've shared for feedback in open-contracting/standard#1449 (comment)

[duncandewhurst]

@jpmckinney, this PR includes a link to a development branch, which will need to be updated when that branch is merged into 1.2-dev and again when 1.2 is released. Happy to hear suggestions for how to better illustrate the guidelines (I do think an illustrative example is important given the potential for confusion about what a real and specific example is).

[jpmckinney]

I added a task to the description of open-contracting/standard#1680 to update the branch to 1.2-dev (we can leave it as staging, as the dev branch will typically always be available).