Added documentation for the sum command

docs/commands/sum.md

@@ -0,0 +1,35 @@
+# sum
+
+This command allows you to calculate the sum of values in a column. 
+
+## Examples
+
+To get the sum of the file sizes in a directory, simply pipe the size column from the ls command to the sum command.
+
+```shell
+> ls | get size | sum
+━━━━━━━━━
+ value
+━━━━━━━━━
+ 51.0 MB
+━━━━━━━━━
+```
+
+Note that sum only works for integer and byte values at the moment, and if the shell doesn't recognize the values in a column as one of those types, it will return an error.
+
+```shell
+> open example.csv
+━━━┯━━━━━━━━━┯━━━━━━━━┯━━━━━━━━━━
+ # │ fruit   │ amount │ quality
+───┼─────────┼────────┼──────────
+ 0 │ apples  │ 1      │ fresh
+ 1 │ bananas │ 2      │ old
+ 2 │ oranges │ 7      │ fresh
+ 3 │ kiwis   │ 25     │ rotten
+━━━┷━━━━━━━━━┷━━━━━━━━┷━━━━━━━━━━
+```
+
+```shell
+> open example.csv | get amount | sum
+error: Unrecognized type in stream: Primitive(String("1"))
+```