Add documentation for externally managed clusters

[enxebre]

/kind documentation

Describe the solution you'd like
Add documentation for externally managed clusters. Also, maybe a developer's note for making sure the reconciliation is blocked in the mapping functions.
#2383

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api-provider-aws version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[sedefsavas]

/help
/milestone v0.7.x

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/help
/milestone v0.7.x

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[Ankitasw]

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