added helper function documentation

Helper-Functions.md

@@ -0,0 +1,67 @@
+`qm` has a number of common helper functions not included in the `evalexpr` crate:
+
+# Mathematics
+
+## Fix
+
+`fix` can be used to round a float to *n* digits.
+
+| Signature           | Description               | Output |
+| ------------------- | ------------------------- | ------ |
+| fix(x: f64, n: i64) | rounds *x* to *n* digits. | f64    |
+
+## Logarithm
+
+`log` can be used to calculate logarithms. Defaults to ln.
+
+| Signature           | Description                              | Output | 
+| ------------------- | ---------------------------------------- | ------ |
+| log(x: num)         | calculates the natural logarithm of *x*  | f64    |
+| log(x: num, n: num) | calculates the base *n* logarithm of *x* | f64    |
+
+## Square Root
+
+`sqrt` is used to calculate square roots.
+
+| Signature    | Description                       | Output |
+| ------------ | --------------------------------- | ------ |
+| sqrt(n: num) | calculates the square root of *n* | f64    |
+| √(n: num)    | "                                 | "      |
+
+# Data Science
+
+## Average
+
+`avg` is used to calculate averages of an arbitrarily large string of numbers.
+
+| Signature          | Description                                    | Output     |
+| ------------------ | ---------------------------------------------- | ---------- |
+| avg(n0: num, ...)  | calculates the average of all numbers provided | i64 or f64 |
+
+# Computer Science
+
+## Radix Conversion
+
+`bin`, `hex`, and `oct` can be used to convert numbers to binary, hexadecimal, and octal strings (and the reverse), respectively.
+
+| Signature   | Description                                                      | Output |
+| ----------- | ---------------------------------------------------------------- | ------ |
+| bin(n: i64) | converts *n* to a string with its binary representation          | str    |
+| bin(b: str) | converts a binary string *b* to the integer it represents        | i64    |
+| hex(n: i64) | converts *n* to a string with its hexadecimal representation     | str    |
+| hex(h: str) | converts a hexidecimal string *h* to the integer it represents   | i64    |
+| oct(n: i64) | converts *n* to a string with its octal representation           | str    |
+| oct(o: str) | converts an octal string *o* to the integer it represents        | i64    |
+
+### Binary
+
+Binary strings will be outputted with the prefix `0b`. It will attempt to parse strings without this prefix, and will handle errors gracefully.
+
+### Hexadecimal
+
+Hexadecimal strings will be outputted with the prefix `0x`. It will attempt to parse strings without this prefix, and will handle errors gracefully.
+
+### Octal
+
+Octal strings will be outputted with the prefix `0o`. It will attempt to parse strings without this prefix, and will handle errors gracefully.
+