Added documentation for units in _commands/tools/units.md

_commands/tools/units.md

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+---
+---
+
+units
+---
+`units` is a unit converter and calculator.
+
+~~~ bash
+$ units
+
+You have: 100 feet
+You want: meters
+	* 30.48
+	/ 0.032808399
+
+You have: 12 gallons
+You want: liters
+	* 45.424941
+	/ 0.022014338
+~~~
+
+<!--more-->
+
+### Using `units`:
+
+On giving the command `units` in your terminal, you are prompted with:
+
+~~~ bash
+You have: 
+~~~
+
+You should then provide either a measurement (`8 liters`) or just the units (`meters`, which is the same as typing `1 meter`). You are then prompted with:
+
+~~~ bash
+You want:
+~~~
+
+You should then provide the units (`feet`, `gallons`, etc.) to which you want the measurement you provided before converted.
+
+If the units provided were compatible (it can't convert `miles` to `milliseconds`!), you will then receive two lines of output; the first will be the answer you are probably looking for, and the second will give the inverse of the factor used to convert between the two units.
+
+### Example:
+
+~~~ bash
+$ units
+
+Currency exchange rates from finance.yahoo.com on 2017-10-31 
+3045 units, 109 prefixes, 109 nonlinear units
+
+You have: 100 feet
+You want: meters
+	* 30.48
+	/ 0.032808399
+~~~
+
+The first line demonstrates that there are 30.48 meters in 100 feet. The second line demonstrates that one meter is .032808399... times the size of 100 feet.
+
+Different messages are displayed when units aren't compatible, when you had `units` perform a reciprocal conversion, etc.
+
+### More Information:
+More information on using `units` can be found in its original [manual](https://www.gnu.org/software/units/manual/units.html).