Improve README

[AlexMaryW]

Issue: #443

Hi, I had a go at improving the README. Do let me know what you think. Thanks! :)

[Shimuuar]

Thanks!

After many years of waiting someone improved rather anemic readme. I added suggestions which basically rewrote description of vectors.

And sorry for the wait.

[Shimuuar]

Do we need Installation section? There's nothing nonstandard it works like any other package

[Shimuuar]

Specifically stream fusion as opposed to build/foldr fusion used for lists.

Suggested change

	An optimisation framework provided in this package, fusion
	is a technique that merges several functions into one and forces
	it to produce only one outcome. Without fusion, your program might
	generate intermediate results for each function separately and
	stall its performance.
	Vector uses stream fusion for optimizations. This is technique
	allows to avoid creation of intermediate data structures. For example
	following expression `sum . filter g . map f` will not allocate temporary vectors
	if compiled with optimizations.

[Shimuuar]

Suggested change

	**Boxed vectors** store each of its elements as a pointer to its value.
	Because we cannot directly access the contents of a boxed vector, they
	are slower in comparison to unboxed vectors.
	**Lazy boxed vectors** (`Data.Vector`) can store any haskell value. Their elements are
	stored as pointers to heap-allocated values and that extra indirection results in worse
	performance.
	**Strict boxed vectors** (`Data.Vector.Strict`) boxed vector which is strict in its elements.

[Shimuuar]

It's more precise to say that they can hold values that are instances of Prim. In fact it can't hold Int# — wrong kind.

Suggested change

	**Primitive vectors** contain elements of primitive type.
	Primitive types can be recognised by the hash sign attached at
	the end of value and/or type’s name, e.g. `3#` or `Int#`. You can read
	more about them [here](https://downloads.haskell.org/~ghc/5.00/docs/set/primitives.html).
	**Primitive vectors** (`Data.Vector.Primitive`). This vector is backed by simple
	byte array and can hold data types which are instances of `Prim` type class.
	This is data types which are represented in memory as sequence of bytes without
	pointer. Think `Int`, `Double`, etc. Usually it's better to use unboxed vectors since
	they have same performance and more general.

[Shimuuar]

In this case name lies.

Suggested change

	**Unboxed vectors** store solely their elements’ values instead of pointers.
	To be unboxed, the elements need to be constant in size. Since we can directly
	access the contents of the unboxed vector, working with them is quite efficient.
	**Unboxed vectors** (`Data.Vector.Unboxed`) is vector which determines representation
	of an array from type of its element. For primitives (`Int`, `Double`, etc) it's backed by primitive
	arrays and for tuples and product types by structure of arrays. Generally it uses unboxed
	representation so it's quite efficient

[Shimuuar]

Suggested change

	**Storable vectors** are pinned, convertible to and from pointers, and
	usable in C functions.
	**Storable vectors** (`Data.Vector.Storable`) are backed by pinned memory. Their primary
	use case is C FFI.

[AlexMaryW]

Thanks for all the suggestions and no worries about the wait! I'll have some time tomorrow to go through your comments and make the necessary changes.

[AlexMaryW]

I read through and adapted your suggestions into the README + deleted the installation section.

[Shimuuar]

I finally got review README again. Few more comments

@lehins @Bodigrim do you any input? After this PR is merged I want to start preparing long overdue 0.13.2.0 release

[Shimuuar]

Suggested change

	This package includes various modules that will allow you to
	work with vectors and use an optimisation framework called [*fusion*](#fusion).
	In this context, vector is an `Int`-indexed array-like data structure with a simpler
	API that can contain any Haskell value. Additionally, its equivalence
	to C-style arrays and optimisation via fusion accelerates vector’s
	performance and makes it a great alternative to list. By installing this
	package, you’ll be able to work with [boxed, unboxed, storable, and primitive
	vectors](#vectors-available-in-the-package) as well as their generic interface.
	An efficient implementation of `Int`-indexed arrays (both mutable and immutable).
	This package provides multiple representations: [boxed, unboxed, storable, and primitive
	vectors](#vectors-available-in-the-package) and generic API which is polymorphic in
	vector type. It also has powerful optimization framework based on [stream-fusion](#fusion)
	which could eliminate intermediate data structures.

I think it would be better to cut down this paragraph a bit.

[Shimuuar]

Do we even need to compare with array. I think at this point it's mostly legacy and barely used