Add table of comparison operators

[Kelly Sovacool, PhD]

Comparison operators return TRUE or FALSE. There are many useful comparison operators in R:

| operator | meaning |
|----------|---------|
| `==`     | equal   |
| `!=`     | not equal |
| `<`      | less than |
| `>`      | greater than |
| `<=`     | less than or equal to |
| `>=`     | greater than or equal to |

Also need & (and) and | (or).

This should go near where they're first introduced.