Markdown-ify BUILDING.

Change-Id: Icd3cba6522ce47a4dfe699204982b5b692d3d62e
Reviewed-on: https://boringssl-review.googlesource.com/5811
Reviewed-by: Adam Langley <agl@google.com>
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+# Building BoringSSL
+
+## Build Prerequisites
+
+  * [CMake] [1] 2.8.8 or later is required.
+
+  * Perl 5.6.1 or later is required. On Windows, [Strawberry Perl] [2] and MSYS
+    Perl have both been reported to work. If not found by CMake, it may be
+    configured explicitly by setting `PERL_EXECUTABLE`.
+
+  * On Windows you currently must use [Ninja] [3] to build; on other platforms,
+    it is not required, but recommended, because it makes builds faster.
+
+  * If you need to build Ninja from source, then a recent version of
+    [Python] [4] is required (Python 2.7.5 works).
+
+  * On Windows only, [Yasm] [5] is required. If not found by CMake, it may be
+    configured explicitly by setting `CMAKE_ASM_NASM_COMPILER`.
+
+  * A C compiler is required. On Windows, MSVC 12 (Visual Studio 2013) or later
+    with Platform SDK 8.1 or later are supported. Recent versions of GCC and
+    Clang should work on non-Windows platforms, and maybe on Windows too.
+
+  * [Go] [6] is required. If not found by CMake, the go executable may be
+    configured explicitly by setting `GO_EXECUTABLE`.
+
+## Building
+
+Using Ninja (note the 'N' is capitalized in the cmake invocation):
+
+    mkdir build
+    cd build
+    cmake -GNinja ..
+    ninja
+
+Using Make (does not work on Windows):
+
+    mkdir build
+    cd build
+    cmake ..
+    make
+
+You usually don't need to run `cmake` again after changing `CMakeLists.txt`
+files because the build scripts will detect changes to them and rebuild
+themselves automatically.
+
+Note that the default build flags in the top-level `CMakeLists.txt` are for
+debugging—optimisation isn't enabled.
+
+If you want to cross-compile then there is an example toolchain file for 32-bit
+Intel in `util/`. Wipe out the build directory, recreate it and run `cmake` like
+this:
+
+    cmake -DCMAKE_TOOLCHAIN_FILE=../util/32-bit-toolchain.cmake -GNinja ..
+
+If you want to build as a shared library, pass `-DBUILD_SHARED_LIBS=1`. On
+Windows, where functions need to be tagged with `dllimport` when coming from a
+shared library, define `BORINGSSL_SHARED_LIBRARY` in any code which `#include`s
+the BoringSSL headers.
+
+### Building for Android
+
+It's possible to build BoringSSL with the Android NDK using CMake. This has
+been tested with version 10d of the NDK.
+
+Unpack the Android NDK somewhere and export `ANDROID_NDK` to point to the
+directory. Clone https://github.com/taka-no-me/android-cmake into `util/`.  Then
+make a build directory as above and run CMake *twice* like this:
+
+    cmake -DANDROID_NATIVE_API_LEVEL=android-9 \
+          -DANDROID_ABI=armeabi-v7a \
+          -DCMAKE_TOOLCHAIN_FILE=../util/android-cmake/android.toolchain.cmake \
+          -DANDROID_NATIVE_API_LEVEL=16 \
+          -GNinja ..
+
+Once you've run that twice, Ninja should produce Android-compatible binaries.
+You can replace `armeabi-v7a` in the above with `arm64-v8a` to build aarch64
+binaries.
+
+## Known Limitations on Windows
+
+  * Versions of CMake since 3.0.2 have a bug in its Ninja generator that causes
+    yasm to output warnings
+
+        yasm: warning: can open only one input file, only the last file will be processed
+
+    These warnings can be safely ignored. The cmake bug is
+    http://www.cmake.org/Bug/view.php?id=15253.
+
+  * CMake can generate Visual Studio projects, but the generated project files
+    don't have steps for assembling the assembly language source files, so they
+    currently cannot be used to build BoringSSL.
+
+
+ [1]: http://www.cmake.org/download/
+ [2]: http://strawberryperl.com/
+ [3]: https://martine.github.io/ninja/
+ [4]: https://www.python.org/downloads/
+ [5]: http://yasm.tortall.net/
+ [6]: https://golang.org/dl/