Started preprocessor docs.

modules/monkey/docs/language/preprocessor.md

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+
+### Preprocessor
+
+Monkey2 include a simple preprocessor that allows you to conditionally compile code depending on a number of build setttings.
+
+The preprocessor supports the following statements: #If, #Else, #ElseIf, #EndIf, #Rem, #End. Preprocessor statements must begin on a new line.
+
+Preprocessor expressions may only use the 'And', 'Or' and comparison operators.
+
+The following symbols may be used in preprocessor expressions:
+
+| Symbol			| Type		| Meaning
+|:------------------|:----------|--------
+|__TARGET__			| String	| The current build target. One of: "windows", "macos", "linux", "android", "ios", "emscripten"
+|__CONFIG__			| String	| The current build config. One of: "release", "debug"
+|__DESKTOP_TARGET__	| Bool		| True if the current build target is windows, macos or linux.
+|__MOBILE_TARGET__	| Bool		| True if the current build target is android or ios.
+|__WEB_TARGET__		| Bool		| True if the current build target is emscripten.
+|__DEBUG__			| Bool		| True if the current build config is debug.
+|__RELEASE__		| Bool		| True if the current build config is release.
+
+For example, to include code in debug builds only, use something like:
+
+```
+#If __DEBUG__
+Print "This code is only included in debug builds."
+#Endif
+```
+
+To include code on desktop or mobile builds, use:
+
+```
+#If __DESKTOP_TARGET__ Or __MOBILE_TARGET__
+Print "This code is only include in desktop and mobile builds."
+#Endif