Recommend proxy-cache usage for valuable external ressources

[benoitbryon]

One recommendation related to https://github.com/benoitbryon/documentation-best-practices/blob/master/docs/channel-communication-flow.txt is to use links rather than repeating existing ressources (documentations, conventions, standards, tutorials, scripts...).

But there is a risk that external content becomes obsolete or disappears:

• content changed
• content moved
• content deleted
• networking issue (offline mode?)

So, a proxy-cache could help:

• backup when content is unavailable
• warning when content is updated (review of changelog may be required)

[benoitbryon]

See also #7