cmd/present: update the instructions for App Engine

The command's documentation mistakingly said that the instructions for
testing and deployment to App Engine were contained in the top-level
README for the tools repository.  As best as I can tell, the top-level
README never contained this information.

This commit adds a configuration fragment and provides sample
instructions for the App Engine workflow.

Change-Id: If89c099e254ef91ca224742aebd81f7d1206d9b8
Reviewed-on: https://go-review.googlesource.com/15685
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/present/doc.go

@@ -7,8 +7,6 @@ Present displays slide presentations and articles. It runs a web server that
 presents slide and article files from the current directory.
 
 It may be run as a stand-alone command or an App Engine app.
-Instructions for deployment to App Engine are in the README of the
-golang.org/x/tools repository.
 
 Usage of present:
   -base="": base path for slide template and static resources
@@ -20,6 +18,33 @@ Usage of present:
 The setup of the Go version of NaCl is documented at:
 https://golang.org/wiki/NativeClient
 
+To use with App Engine, copy the tools/cmd/present directory to the root of
+your application and create an app.yaml file similar to this:
+
+    application: [application]
+    version: [version]
+    runtime: go
+    api_version: go1
+
+    handlers:
+    - url: /favicon.ico
+      static_files: present/static/favicon.ico
+      upload: present/static/favicon.ico
+    - url: /static
+      static_dir: present/static
+      application_readable: true
+    - url: /.*
+      script: _go_app
+
+    # nobuild_files is a regexp that identifies which files to not build.  It
+    # is useful for embedding static assets like code snippets and preventing
+    # them from producing build errors for your project.
+    nobuild_files: [path regexp for talk materials]
+
+Present then can be tested in a local App Engine environment with
+
+    goapp serve
+
 Input files are named foo.extension, where "extension" defines the format of
 the generated output. The supported formats are:
 	.slide        // HTML5 slide presentation