Add installation instructions for pip

Several issues report problems with using `pip install basemap`. A workaround is using the latest version from Github.

Some issues with pip problems:

Author: heitorPB

README.md

@@ -100,6 +100,15 @@ prompt.
 To run all the examples (except those that have extra dependencies
 or require an internet connection), execute 'python run_all.py'.
 
+An alternative method is using `pip`:
+
+```
+pip install git+https://github.com/matplotlib/basemap.git
+```
+
+You migh want to run pip with administrative privilegies (e.g. `sudo pip`) or
+perform an user only installation (e.g. `pip install --user`).
+
 ## Contact
 
 Ben Root <[email protected]>