add more information in docs

Author: juandspy

README.md

@@ -11,10 +11,68 @@ git clone https://github.com/juandspy/monkey-lang.git
 go run main.go
 ```
 
-and start writing Monkey lang code.
+and start writing Monkey lang code. The results of the statements will be printed in the stdout. For example:
+
+```
+>> 3 * 7
+21
+>> let x = 3 * 7
+>> x
+21
+```
+
+Note that some statements like variable bindings don't print anything in the stdout.
 
 ## Language specs
 
+### Types
+
+There are 5 types supported:
+
+- Booleans: `true` or `false`
+- Integers: `1`, `-1`, `12345`...
+- Strings: `"Hello World"`
+- Arrays: `[1, 2, 3]`. You can access a given position of an array by using indexes: `[1, 2, 3][1]` or `myArray[1]`.
+- Hashes: `{"a": 1, 5: "test", true: "bool"}`
+
 ### Operators
 
-- Bang (!): it takes any input and returns the opposite. For example `!true = false` and `!5 = false`, as `5` acts as "truthy". However, `!!5` would be `true` as it's the same as `!false`.
+#### Prefix expressions
+
+- Bang (`!`): it takes any input and returns the opposite. For example `!true = false` and `!5 = false`, as `5` acts as "truthy". However, `!!5` would be `true` as it's the same as `!false`.
+- Minus (`-`): changes the sign of an integer e.g. `-5`.
+
+#### Infix expressions
+
+- Arithmetic expressions: `(1 + 2) * 3 / 4`
+- Comparisons: `3 != 2`
+- Conditionals: `if (3 == 3) {"equals"} else {"not equals"}`
+
+### Variables
+
+You can define a variable by using `let` statements, e.g. `let x = 3`. You can also bind expressions: `let x = 3 * 7`.
+
+#### Functions
+
+You can bind functions to variables using the `let` statement:
+
+```
+let sum = fn(x, y) {return x + y}; sum(1, 2)
+```
+
+You can also build recursive functions:
+```
+>> let fib = fn(n) { if (n < 2) { return n; } else {return fib(n-1) + fib(n-2); } }
+>> fib(20)
+6765
+```
+
+#### Builtin functions
+
+There is a set of builtin functions available which are defined in [builtins.go](evaluator/builtins.go):
+- `len`: returns the length of a string or array.
+- `first`: returns the first element of an array.
+- `last`: returns the last element of an array.
+- `rest`: returns all the elements except the first one.
+- `push`: appends an item to an array.
+- `puts`: output to stdout.