Improve virtualenv command suggestion in readme

The Python in the virtualenv doesn't know about pcbnew module.
When installed normally, this can be provided when setting up the
virtualenv. This lets you avoid messing about with PYTHONPATH.

The old instructions apply when KiCad is not installed to the
system (common when developing)

Author: kasbah

README.md

@@ -18,27 +18,32 @@ datasheet.
 KiPlot lets you do this.
 
 As a side effect of providing a scriptable plot driver for KiCad, KiPlot also
-allows functional testing of KiCad plot functions, which would otherwise be 
+allows functional testing of KiCad plot functions, which would otherwise be
 somewhat unwieldy to write.
 
 ## Developing
 
-Set up a virtualenv:
+### Set up a virtualenv (if you installed KiCad normally)
+
+If you installed KiCad from a package manager, or you used `make install`,
+you probably have the packages and libraries on you system paths.
 
 ```
-virtualenv --python /usr/bin/python2.7 ~/venv/kiplot
-source ~/venv/kiplot/bin/activate
+virtualenv --python /usr/bin/python2.7 --system-site-packages ~/venv/kiplot
 ```
 
-Install `kiplot` with `pip -e`:
+### Set up a virtualenv (if you installed KiCad locally)
+
+
+If the `pcbnew` Python package is *not* installed at a system level (e.g. if
+you are building locally and not installing to the system, you should not
+need any system packages:
 
 ```
-cd path/to/kiplot
-pip install -e .
+virtualenv --python /usr/bin/python2.7 ~/venv/kiplot
 ```
-
-This doesn't include the `pcbnew` Python package - that is assumed to
-be accessible to the program. You might need to add it to the `PYTHONPATH`.
+However, you must make sure `pcbnew` is accessible to KiPlot.
+You might need to add it to the `PYTHONPATH`.
 
 You might also need to set `LD_LIBRARY_PATH` (you need to be able to load
 `libkicad_3dsg.so`).
@@ -50,7 +55,20 @@ export PYTHONPATH=~/local/kicad/lib/python2.7/site-packages
 export LD_LIBRARY_PATH=~/local/kicad/lib64
 ```
 
-If you've installed "normally", you should not need to do this.
+### Install KiPlot to the virtualenv
+
+Activate the virtualenv:
+
+```
+source ~/venv/kiplot/bin/activate
+```
+
+Install `kiplot` with `pip -e`:
+
+```
+cd path/to/kiplot
+pip install -e .
+```
 
 ## Testing