Merge to XFA: [Docs] Add wiki content to Markdown docs

docs/build.md

+# Building PDFium
+
+## Prerequisites
+
+Get the chromium depot tools via the instructions at
+http://www.chromium.org/developers/how-tos/install-depot-tools (this provides
+the gclient utilty needed below).
+
+Also install Python, Subversion, and Git and make sure they're in your path.
+
+Optionally, you may want to install the [Ninja](http://martine.github.io/ninja/)
+build system (recommended) rather than using your platform-specific build
+system.
+
+## Get the code
+
+```
+mkdir pdfium
+cd pdfium
+gclient config --name . --unmanaged https://pdfium.googlesource.com/pdfium.git
+gclient sync
+```
+
+## Generate the build files
+
+Now we use the GYP library to generate the build files.
+
+At this point, you have two options. The first option is to use the [Ninja]
+(http://martine.github.io/ninja/) build system. This is the default as of
+mid-September, 2015. Previously, the second option was the default. Most PDFium
+developers use Ninja, as does our [continuous build system]
+(http://build.chromium.org/p/client.pdfium/).
+
+On Windows: `build\gyp_pdfium
+` For all other platforms: `build/gyp_pdfium
+`
+
+The second option is to generate platform-specific build files, i.e. Makefiles
+on Linux, sln files on Windows, and xcodeproj files on Mac. To do so, set the
+GYP\_GENERATORS environment variable appropriately (e.g. "make", "msvs", or
+"xcode") before running the above command.
+
+## Building the code
+
+If you used Ninja, you can build the sample program by: `ninja -C out/Debug
+pdfium_test
+` You can build the entire product (which includes a few unit tests) by: `ninja
+-C out/Debug
+`
+
+If you're not using Ninja, then building is platform-specific.
+
+On Linux: `make pdfium_test
+`
+
+On Mac, open build/all.xcodeproj
+
+On Windows, open build\all.sln
+
+## Running the sample program
+
+The pdfium\_test program supports reading, parsing, and rasterizing the pages of
+a .pdf file to .ppm output image files (windows supports two other formats, and
+.png support is available for all platforms in an alternate branch (see branches
+section below)). For example: `out/Debug/pdfium_test --ppm path/to/myfile.pdf
+`
+
+## Waterfall
+
+The current health of the source tree can be found at
+http://build.chromium.org/p/client.pdfium/console
+
+## Branches
+
+There is a branch for a forthcoming feature called XFA that you can get by
+following the steps above, then: `git checkout origin/xfa build/gyp_pdfium ninja
+-C out/Debug
+`
+
+The XFA version of the sample pdfium\_test program supports rasterizing to .png
+format files. For example: `out/Debug/pdfium_test --png path/to/myfile.pdf
+`