docs: update top level readme: user's guide; demo mode refs

Added link to really impatient section of insallation.txt that
includes detailed directions on running demo mode.

Included section referencing the user's guide.

Renamed "Usage and Other Information" to "Other Information" with
addition of usage section.

Removed uneeded extra newline.

Author: rouilj

README.txt

@@ -41,6 +41,12 @@ a web server. To reset demo instance::
 
    python demo.py nuke
 
+For details see the "For the Really Impatient" section of the
+installation document at:
+
+   https://roundup-tracker.org/docs/installation.html#for-the-really-impatient
+
+for details on running demo mode or using the docker demo mode.
 
 Tracker Home
 ------------
@@ -50,6 +56,12 @@ directory where all your tracker data is stored. This directory is
 created every time when new tracker is initialized and includes
 tracker configuration, database, template, schema and extensions.
 
+Using Roundup
+=============
+
+Please see the user's guide at: 
+
+  https://roundup-tracker.org/docs/installation.html#for-the-really-impatient
 
 Installation
 ============
@@ -72,15 +84,14 @@ Upgrading
 
 Please see "doc/upgrading.txt".
 
-
 Security Issues
 ===============
 
 Please see "doc/security.txt" for directions on reporting security issues.
 
 
-Usage and Other Information
-===========================
+Other Information
+=================
 
 Start with the index.txt file in the "doc" directory. These
 documentation files are written in reStructedText, which can be