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Add a section about fixity declarations for functions (#381)

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`@@ -267,6 +267,45 @@ In PureScript, operator sections look a little bit different.`
`267`	`267`	`(_ ^ 2)`
`268`	`268`	```
`269`	`269`
	`270`	`+## Fixity declarations for functions`
	`271`	`+`
	`272`	+In Haskell, functions can have fixity declarations. For example in `base`:
	`273`	`+`
	`274`	+```haskell
	`275`	+infix 4 `elem`, `notElem`
	`276`	+infixl 7 `quot`, `rem`, `div`, `mod`
	`277`	+```
	`278`	`+`
	`279`	`+However fixity declarations are only supported while declaring [operator aliases](Syntax.md#binary-operators) for functions in PureScript. Functions are always left associative and have the highest precedence when used as infix operators, which means that:`
	`280`	`+`
	`281`	+```
	`282`	+x * y `mod` z
	`283`	+```
	`284`	`+`
	`285`	`+parses as:`
	`286`	`+`
	`287`	+```haskell
	`288`	+((x * y) `mod` z)
	`289`	+```
	`290`	`+`
	`291`	`+in Haskell, but as:`
	`292`	`+`
	`293`	+```purescript
	`294`	+(x * (y `mod` z))
	`295`	+```
	`296`	`+`
	`297`	`+in PureScript, and those expressions may yield different values:`
	`298`	`+`
	`299`	+```purescript
	`300`	+> ((2 * 3) `mod` 5)
	`301`	`+1`
	`302`	+```
	`303`	`+`
	`304`	+```purescript
	`305`	+> (2 * (3 `mod` 5))
	`306`	`+6`
	`307`	+```
	`308`	`+`
`270`	`309`	`## Extensions`
`271`	`310`
`272`	`311`	`The PureScript compiler does not support GHC-like language extensions. However, there are some "built-in" language features that are equivalent (or at least similar) to a number of GHC extensions. These currently are:`

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