Added a word frequency as a how-to.

Returns the frequency of occorance of words in an SArray of documents

Example:

docs = gl.SArray(['The quick', 'brown fox',
                         'jumps over the', 'lazy dog'])
docs_count = get_word_frequency(docs)
print docs_count

# +-------+-------+-----------+
# |  word | count | frequency |
# +-------+-------+-----------+
# | brown |   1   |    0.25   |
# |  lazy |   1   |    0.25   |
# |  dog  |   1   |    0.25   |
# | quick |   1   |    0.25   |
# | jumps |   1   |    0.25   |
# |  the  |   2   |    0.5    |
# |  fox  |   1   |    0.25   |
# |  over |   1   |    0.25   |
# +-------+-------+-----------+