update getting started (AtsushiSakai#1136)

Co-authored-by: Atsushi Sakai <[email protected]>

Author: AtsushiSakai

docs/modules/0_getting_started/3_how_to_contribute_main.rst

@@ -1,4 +1,4 @@
-How To Contribute
+How to contribute
 =================
 
 This document describes how to contribute this project.

docs/modules/0_getting_started/4_how_to_read_textbook_main.rst

@@ -3,4 +3,12 @@
 How to read this textbook
 --------------------------
 
-TBD
+This document is structured to help you learn the fundamental concepts
+behind each sample code in PythonRobotics.
+
+If you already have some knowledge of robotics technologies, you can start
+by reading any document that interests you.
+
+However, if you have no prior knowledge of robotics technologies, it is
+recommended that you first read the :ref:`Introduction` section and then proceed
+to the documents related to the technical fields that interest you.

docs/modules/1_introduction/introduction_main.rst

@@ -1,4 +1,4 @@
-.. _introduction:
+.. _Introduction:
 
 Introduction
 ============