Add some C++ documentation

Fixes #32.

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MOS_MIGRATED_REVID=92358818

site/docs/cpp.md

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+---
+layout: documentation
+---
+
+Use fully qualified include paths
+=================================
+
+Includes are relative to the root of your workspace. For example, suppose
+you have the following directory structure:
+
+```
+[workspace]/
+    WORKSPACE
+    a/
+        BUILD
+        a.h
+        a.cc
+    b/
+        BUILD
+        b.h
+        b.cc
+        main.cc
+```
+
+If _b/main.cc_ needs to include b.h then we'd create the following _b/BUILD_
+file:
+
+```python
+cc_library(
+    name = "b",
+    srcs = ["b.cc"],
+    hdrs = ["b.h"],
+)
+
+cc_binary(
+    name = "main",
+    srcs = ["main.cc"],
+    deps = [":b"],
+)
+```
+
+_b/main.cc_ would have the following include statement:
+
+```cpp
+#include "b/b.h"
+```
+
+Note that the full path from the package root is used. If we want _b/main.cc_ to
+also depend on _a/a.h_, we'd add the rule to _a/BUILD_:
+
+```python
+cc_library(
+    name = "a",
+    srcs = ["a.cc"],
+    hdrs = ["a.h"],
+    visibility = ["//b:__pkg__"],
+)
+```
+
+Then we'd add a dependency to _b/BUILD_:
+
+```python
+cc_binary(
+    name = "main",
+    srcs = ["main.cc"],
+    deps = [
+        ":b",
+        "//a",
+    ],
+)
+```
+
+And the following include to _b/main.cc_:
+
+```cpp
+#include "a/a.h"
+```
+
+_b/main.cc_ will then be able to access symbols from _a/a.h_ or _b/b.h_.
+
+Transitive includes
+===================
+
+If a file includes a header then the file's rule should depend on that header's
+library.  Conversely, only direct dependencies need to be specified as
+dependencies.  For example, suppose _sandwich.h_ includes _bread.h_ and
+_bread.h_ includes _flour.h_.  _sandwich.h_ doesn't include _flour.h_ (who wants
+flour in their sandwich?), so the BUILD file would look like:
+
+```python
+cc_library(
+    name = "sandwich",
+    srcs = ["sandwich.cc"],
+    hdrs = ["sandwich.h"],
+    deps = [":bread"],
+)
+
+cc_library(
+    name = "bread",
+    srcs = ["bread.cc"],
+    hdrs = ["bread.h"],
+    deps = [":flour"],
+)
+
+cc_library(
+    name = "flour",
+    srcs = ["flour.cc"],
+    hdrs = ["flour.h"],
+)
+```
+
+This expresses that the "sandwich" library depends on the "bread" library,
+which depends on the "flour" library.
+
+Adding include paths
+====================
+
+Sometimes you cannot (or do not want to) base include paths at the workspace
+root. Existing libaries might already have a include directory that doesn't
+match its path in your workspace.  For example, suppose you have the following
+directory structure:
+
+```
+[workspace]/
+    WORKSPACE
+    third_party/
+        some_lib/
+            include/
+                some_lib.h
+            BUILD
+            some_lib.cc
+```
+
+Bazel will expect _some\_lib.h_ to be included as
+`third_party/some_lib/include/some_lib.h`, but suppose _some\_lib.cc_ includes
+`"include/some_lib.h"`.  To make that include path valid,
+_third\_party/some\_lib/BUILD_ will need to specify that the _some\_lib/_
+directory is an include directory:
+
+```python
+cc_library(
+    name = "some_lib",
+    srcs = ["some_lib.cc"],
+    hdrs = ["some_lib.h"],
+    includes = ["."],
+)
+```
+
+This is especially useful for external dependencies, as their header files
+must otherwise be included with an "external/[repository-name]/" prefix.
+
+Including external libraries: an example
+========================================
+
+Suppose you are using [Google Test](https://code.google.com/p/googletest/). You
+can use one of the `new_` repository functions in the WORKSPACE file to create
+the repository:
+
+```python
+new_http_archive(
+    name = "gtest-repo",
+    url = "https://googletest.googlecode.com/files/gtest-1.7.0.zip",
+    sha256 = "247ca18dd83f53deb1328be17e4b1be31514cedfc1e3424f672bf11fd7e0d60d",
+    build_file = "gtest.BUILD",
+)
+
+bind(
+    name = "gtest/main",
+    actual = "@gtest-repo//:main",
+)
+```
+
+Then create _gtest.BUILD_, a BUILD file to use to compile Google Test.
+Google Test has several "special" requirements that make its `cc\_library` rule
+more complicated:
+
+* It has a .cc file that `#include`s all of the other files in _src/_, so we
+  need to exclude it from the compile or we'll get link errors for duplicate
+  symbols.
+* It uses header files that relative to the _include/_ directory
+  (`"gtest/gtest.h"`), so we must add that directory the includes.
+* It uses "private" header files in src/, so we add "." to the includes so it
+  can `#include "src/gtest-internal-inl.h"`.
+* It needs to link in pthread, so we add that as a `linkopt`.
+
+The final rule looks like this:
+
+```python
+cc_library(
+    name = "main",
+    srcs = glob(
+        ["src/*.cc"],
+        exclude=["src/gtest-all.cc"]
+    ),
+    hdrs = glob(["include/**/*.h"]),
+    includes = [
+        ".",
+        "include"
+    ],
+    linkopts = ["-pthread"],
+    visibility = ["//visibility:public"],
+)
+```
+
+Now `cc_` rules can depend on `//external:gtest/main`.
+
+For example, we could create a test such as:
+
+```cpp
+#include "gtest/gtest.h"
+
+TEST(FactorialTest, Negative) {
+  EXPECT_EQ(1, 1);
+}
+```
+
+Then create a BUILD file for your tests:
+
+```python
+cc_test(
+    name = "my_test",
+    srcs = ["my_test.cc"],
+    deps = ["//external:gtest/main"],
+)
+```
+
+You can then use `bazel test` to run the test.