Add If Keyword

If-Keyword.md

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+Keyword: `if`
+
+Syntax: `if $bool %line`
+
+Time Complexity: 
+
+## Overview
+
+The `if` keyword allows you to jump to a label or line if a condition is true. It is a core part of the ground programming language, as it would not be [Turing Complete](https://en.wikipedia.org/wiki/Turing_completeness) without.
+
+## Example usage
+```grnd
+@label
+    if false %label
+```
+The second line checks if `false` is true. Obviously this is false, so nothing happens and program execution continues.
+
+However, consider this code:
+```grnd
+@label
+    if true %label
+```
+The second line checks if `true` is true. This is true, so the program jumps back to `@label`, creating an infinite loop.
+
+A more practical example would be to repeat some code a certain amount of times, e.g. 5
+
+```grnd
+set &loop_counter 0
+@start_loop
+    equal $loop_counter 5 &cond # if loop_counter equals 5, set cond to true, otherwise set cond to false
+    if $cond %end_loop # If cond (i.e. if loop_counter equals 5), exit the loop
+    # Do some code
+    add $loop_counter 1 &loop_counter # Increment loop_counter
+    jump %start_loop # Repeat
+@exit_loop
+```