Add overall Blueprint documentation

Change-Id: I27f0f010c32b3abe0107e69af830716ab728485a

doc.go

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+// Copyright 2015 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Blueprint is a meta-build system that reads in Blueprints files that describe
+// modules that need to be built, and produces a Ninja 
+// (http://martine.github.io/ninja/) manifest describing the commands that need
+// to be run and their dependencies.  Where most build systems use built-in
+// rules or a domain-specific langauge to describe the logic how modules are
+// converted to build rules, Blueprint delegates this to per-project build logic
+// written in Go.  For large, heterogenous projects this allows the inherent
+// complexity of the build logic to be maintained in a high-level language,
+// while still allowing simple changes to individual modules by modifying easy
+// to understand Blueprints files.
+//
+// Blueprint uses a bootstrapping process to allow the code for Blueprint,
+// the code for the build logic, and the code for the project being compiled
+// to all live in the project.  Dependencies between the layers are fully 
+// tracked - a change to the logic code will cause the logic to be recompiled,
+// regenerate the project build manifest, and run modified project rules.  A
+// change to Blueprint itself will cause Blueprint to rebuild, and then rebuild
+// the logic, etc.
+//
+// A Blueprints file is a list of modules in a pseudo-python data format, where
+// the module type looks like a function call, and the properties of the module
+// look like optional arguments.  For example, a simple module might look like:
+//
+// cc_library(
+//     name = "cmd",
+//     srcs = [
+//         "main.c",
+//     ],
+//     deps = [
+//         "libc",
+//     ],
+// )
+//
+// subdirs = ["subdir1", "subdir2"]
+//
+// The modules from the top level Blueprints file and recursively through any
+// subdirectories listed by the "subdirs" variable are read by Blueprint, and
+// their properties are stored into property structs by module type.  Once
+// all modules are read, Blueprint calls any registered Mutators, in
+// registration order.  Mutators can visit each module top-down or bottom-up,
+// and modify them as necessary.  Common modifications include setting
+// properties on modules to propagate information down from dependers to
+// dependees (for example, telling a module what kinds of parents depend on it),
+// or splitting a module into multiple variants (for example, one per
+// architecture being compiled).  After all Mutators have run, each module is
+// asked to generate build rules based on property values, and then singletons
+// can generate any build rules from the output of all modules.
+//
+// The per-project build logic defines a top level command, referred to in the
+// documentation as the "primary builder".  This command is responsible for
+// registering the module types needed for the project, as well as any
+// singletons or mutators, and then calling into Blueprint with the path of the
+// root Blueprint file.
+package blueprint