Update to documentation for position specifiers

README.md

@@ -225,4 +225,23 @@ This command remeshes the atoms/elements within the group to the new element siz
 -ow
 ```
 
-If this option is passed then all files are automatically overwritten without asking the user.
+If this option is passed then all files are automatically overwritten without asking the user.
+
+## Position Specification
+
+Specifying positions in cacmb can be done through a variety of ways. Examples of each format is shown below.
+
+`val` - Where `val` is a number, then that value in Angstroms is used as the position. As an example, `11.1` would be read in as a position of 11.1 $\AA$.
+
+`-inf` - This specifies the lower box boundary in the specific dimension. The vector `-inf -inf -inf`  specifies the bottom corner of the simulation cell which also acts as the simulation cell origin. The vector `-inf 10 3` instead puts only the x position at the simulation cell origin.
+
+`inf` - Similar to `-inf` but references the upper boundary of the box in that dimension
+
+`inf-val` - Using a minus sign reduces the position from the **upper boundary** by `val`. `inf-10` would be at a distance of $10 \AA$ from the upper boundary in that dimension.
+
+`inf+val` - This increases the position from the **lower boundary**. `inf+10` would be a position $10\AA$ from the lower boundary within the cell.
+
+`inf*val` - This gives you a fractional position in the simulation cell. As an example `inf*0.5` gives you the center point of the simulation cell. 
+
+`rand` - Returns a random position that lies within the simulation cell.
+