* Add overview doc

docs/guide/overview.rst

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+Overview
+========
+
+What and Why
+------------
+
+spaCy is a lightning-fast, full-cream NLP tokenizer, tightly coupled to a
+global vocabulary store.
+
+Most tokenizers give you a sequence of strings. That's barbaric.
+Giving you strings invites you to compute on every *token*, when what
+you should be doing is computing on every *type*.  Remember
+`Zipf's law <http://en.wikipedia.org/wiki/Zipf's_law>`_: you'll
+see exponentially fewer types than tokens.
+
+Instead of strings, spacy gives you Lexeme IDs, from which you can access
+an excellent set of pre-computed orthographic and distributional features:
+
+::
+
+    >>> from spacy import en
+    >>> apples, are, not, oranges, dots = en.tokenize(u"Apples aren't oranges...")
+    >>> en.is_lower(apples)
+    False
+    # Distributional features calculated from large corpora
+    # Smoothed unigram log probability
+    >>> en.prob_of(are) > en.prob_of(oranges)
+    True
+    # After POS tagging lots of text, is this word ever a noun?
+    >>> en.can_tag(are, en.NOUN)
+    False
+    # Is this word always title-cased?
+    >>> en.often_title(apples)
+    False
+
+Accessing these properties is essentially free: the Lexeme IDs are actually
+memory addresses that point to structs --- so the only cost is the Python
+function call overhead.  If you call the accessor functions from Cython,
+there's no overhead at all.
+
+Benchmark
+---------
+
+Because it exploits Zipf's law, spaCy is much more efficient than
+regular-expression based tokenizers.  See Algorithm and Implementation Details
+for an explanation of how this works.
+
++--------+-------+--------------+--------------+
+| System | Time	 | Words/second | Speed Factor |
++--------+-------+--------------+--------------+
+| NLTK	 | 6m4s  | 89,000       | 1.00         |
++--------+-------+--------------+--------------+
+| spaCy	 | 9.5s	 | 3,093,000	| 38.30        |
++--------+-------+--------------+--------------+
+
+The comparison refers to 30 million words from the English Gigaword, on
+a Maxbook Air.  For context, calling string.split() on the data completes in
+about 5s.
+
+Pros and Cons
+-------------
+
+Pros:
+
+- All tokens come with indices into the original string
+- Full unicode support
+- Extensible to other languages
+- Batch operations computed efficiently in Cython
+- Cython API
+- numpy interoperability
+
+Cons:
+
+- It's new (released September 2014)
+- Higher memory usage (up to 1gb)
+- More conceptually complicated
+- Tokenization rules expressed in code, not as data
+