Add instructions for how to set up in a virtualenv

Using a virtualenv is much better than globally installing all of the
project's dependencies, as it doesn't pollute the global package space,
and allows us to work just within the project folder.

.gitignore

@@ -36,3 +36,4 @@ docs/_build/*
 deps
 ghostdriver.log
 seahub/thumbnail/thumb/*
+.virtualenv

README.markdown

@@ -17,9 +17,14 @@ You can grab souce code from GitHub.
 
     $ git clone git://github.com/haiwen/seahub.git
 
+Set up a virtualenv to install dependencies locally:
+
+    $ virtualenv .virtualenv
+    $ . .virtualenv/bin/activate
+
 Install python libraries by pip:
 
-    pip install -r requirements.txt
+    $ pip install -r requirements.txt
 
 
 Configuration
@@ -38,7 +43,8 @@ Run and Verify
 
 Run as:
 
-    ./run-seahub.sh.template
+    $ . .virtualenv/bin/activate
+    $ ./run-seahub.sh.template
 
 Then open your browser, and input `http://localhost:8000/`, there should be a Login page. You can create admin account using `seahub-admin.py` script under `tools/` directory.