Add 'Reference'

Reference.md

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+This is the reference for sandwich's language features.
+
+# Opcodes
+
+`sandwich` is comprised of *opcodes*. The syntax looks like this:
+```
+oArguments
+```
+
+The currently available opcodes are as follows:
+
+- `p` - print out the arguments
+    - `pHello World!` will print out `Hello World!`.
+- `a`, `s`, `m`, `d` - add, subtract, multiply, and divide, respectively
+    - the arguments look like the following:
+    - `<num1>-<num2>`
+    - For example: `a10-10` will print out `20`.
+    - the dash in the middle is a separator between the two input numbers.
+    - These opcodes will error out if either of the arguments is not a number.
+- `l` - declare a variable
+    - `<variable name><variable value>`
+    - the name of the variable must be only 1 character
+    - the value can be any length.
+    - if the first character of the value is a `<`, sandwich will grab input from the user, using everything after the `<` as a prompt.
+    - For example, if your code says `ln<Enter your name: `, sandwich will prompt the user with `Enter your name: `  and wait for the user to input.
+    - Then, it will assign the inputted text to the variable `n`.
+    - Note:
+        - To use a variable, use the $ character.
+        - For example, `p$v` will print out the value of variable `v`.
+        - If `w` is set to `World`, `pHello, $w!` will print out `Hello, World!`
+        - Two variables cannot share the same name. If you try to redeclare a variable, the old value will be overwritten.
+- `f` - declare a function
+    - The arguments are the same as declaring a variable.
+    - the body of the function has to be one of the math operators.
+    - Note:
+        - To use a variable, use the * character.
+        - For example, `p*f` will print out the result of function `f`.
+        - Functions will only get evaluated *when they're called*.
+        - This means, if you use variable `v` in a function, but `v` gets redefined between the function declaration and when you use it, the function will use the *new* value of `v`, and not the value from when the function was defined.
+        - Also, functions are re-evaluated *for each call*. 
+        - This means that if the function depend on an outside variable and the variable changes frequently, the result of the function will also change frequently.
+        - Like variables, two functions cannot share the same name. If you try to redeclare a function, the old function will get overwritten.
+- `i` - import variables and functions from another file
+    - The arguments look like the following:
+    - `<filename>[-optional-list-of-functions-or-variables-to-import]`
+    - the list of values should be separated by a `-`. Each value should have the following syntax:
+    - `<f or v, function or variable, respectively><name of function/variable>`