added installation instructions

web/install.rst

+Installing NLTK
+===============
+
+Mac/Unix
+--------
+
+#. Open ``Finder>Applications>Utilities>Terminal`` and type ``python -V`` to find out what version of Python is installed
+#. Install Setuptools: Download the corresponding version of Setuptools from
+   http://pypi.python.org/pypi/setuptools (scroll to the bottom, and pick the filename that contains the right version number and which has the extension .egg).  Install it by typing ``sudo sh Downloads/setuptools-...egg``, giving the location of the downloaded file.
+#. Install Pip: run ``sudo easy_install pip``
+#. Install Numpy: run ``sudo pip install numpy --upgrade``
+#. Install NLTK: run ``sudo pip install nltk --upgrade``
+#. Test installation: run ``python`` then type ``import nltk``
+
+Windows
+-------
+
+These instructions assume that you do not already have Python installed on your machine.
+If you do, you can skip to the final step and just install NLTK.
+
+32-bit binary installation
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+#. Install Python: http://www.python.org/download/releases/2.7.2/
+#. Install Numpy (optional): http://sourceforge.net/projects/numpy/files/NumPy/1.6.1/numpy-1.6.1-win32-superpack-python2.7.exe
+#. Install NLTK: http://pypi.python.org/pypi/nltk
+#. Test installation: ``Start>Python27``, then type ``import nltk``
+
+Source installation (for 32-bit or 64-bit Windows)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+#. Install Python: http://www.python.org/download/releases/2.7.2/
+#. Install Numpy (optional): http://www.lfd.uci.edu/~gohlke/pythonlibs/#numpy
+#. Install Setuptools: http://pypi.python.org/packages/2.7/s/setuptools/setuptools-0.6c11.win32-py2.7.exe
+#. Install Pip: ``Start>Run... c:\Python27\Scripts\easy_install pip``
+#. Install NLTK: ``Start>Run... pip install nltk``
+#. Test installation: ``Start>Python27``, then type ``import nltk``
+