Added message that setup.py develop is required to build the docs

docs/conf.py

@@ -43,12 +43,20 @@ master_doc = 'index'
 project = u'Flask'
 copyright = u'2010, Armin Ronacher'
 
-import pkg_resources
-
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
 # built documents.
-release = __import__('pkg_resources').get_distribution('Flask').version
+import pkg_resources
+try:
+    release = pkg_resources.get_distribution('Flask').version
+except pkg_resources.DistributionNotFound:
+    print 'To build the documentation, The distribution information of Flask'
+    print 'Has to be available.  Either install the package into your'
+    print 'development environment or run "setup.py develop" to setup the'
+    print 'metadata.  A virtualenv is recommended!'
+    sys.exit(1)
+del pkg_resources
+
 if 'dev' in release:
     release = release.split('dev')[0] + 'dev'
 version = '.'.join(release.split('.')[:2])