Document formulas for calculating resource requirements

[cwjohnston]

In order to properly provision Sensu Go deployments, we believe it's necessary to calculate

• Events per second rate for a given configuration
• IOPS required to sustain events per second
• Megabytes-per-second etcd writes
• etcd database size

Given documented formulas for these we should be able to implement calculators that implementors can use to understand what is required for their deployment.