Update contributor guidlines for authoring examples

[duncandewhurst]

From open-contracting/standard#1449 (comment):

Update the contributor guidelines for authoring examples as follows:

• Explain what is meant by 'real' (i.e. featuring real organisations and plausible and coherent field values, not being real data from an OCDS publisher)
• Examples ought to feature the minimum level of complexity and minimum set of fields with respect to the concepts they are supposed to illustrate.
• Examples on the same page ought to follow a common thread/context/scenario.