Add instructions on how to generate the configure script.

README.txt

@@ -5,6 +5,18 @@ https://developers.google.com/protocol-buffers/
 C++ Installation - Unix
 =======================
 
+If you get the source from github, you need to generate the configure script
+first:
+
+  $ ./autogen.sh
+
+This will download gtest source (which is used for C++ Protocol Buffer
+unit-tests) to the current directory and run automake, autoconf, etc.
+to generate the configure script and various template makefiles.
+
+You can skip this step if you are using a release package (which already
+contains gtest and the configure script).
+
 To build and install the C++ Protocol Buffer runtime and the Protocol
 Buffer compiler (protoc) execute the following: