[DOC]: Add conda to install guide

Add a section to the install guide in the Iris docs that is about installing Iris using conda.
It's not wrong, in fact it works just fine. In fact I'm going to argue that is _more_ right than 
```
conda install -c scitools iris
```
because on [binstar](https://binstar.org/scitools) we clearly call ourselves `SciTools` (note camel case). If instead we chose to tell people to install Iris using
```
conda install -c https://conda.binstar.org/scitools iris
```
 then not having camel case would be appropriate because that literally is our binstar URL.