Add Linux quick start docs

Preview: http://skiadocs.com:8000/user/quick/linux?cl=846523002

BUG=skia:

Review URL: https://codereview.chromium.org/846523002

site/user/quick/linux.md

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+Linux
+=====
+
+Prerequisites
+-------------
+
+On a Ubuntu 12.04 (Precise) or Ubuntu 14.04 (Trusty) system, you can run
+`tools/install_dependencies.sh`, which will install the needed packages.  On
+Ubuntu 12.04, you will need to install the`ninja` build tool separately, which
+comes with Chromium's `depot_tools`.
+
+To contribute changes back to Skia, you will need `git-cl`, which comes with Chromium's `depot_tools`.
+
+(If you use another Linux distribution, please consider contributing back
+instructions for installing the required packages — we can then incorporate
+that knowledge into the `tools/install_dependencies.sh` tool.)
+
+Make sure the following have been installed:
+
+  * Chromium depot_tools: http://www.chromium.org/developers/how-tos/depottools
+  * A C++ compiler (typically gcc)
+  * python 2.7.x
+    * suggested Ubuntu packages: python2.7, python2.7-dev
+  * The FreeType and Fontconfig font engines
+    * suggested Ubuntu packages: libfreetype6, libfreetype6-dev , libfontconfig , libfontconfig-dev e.g., `sudo apt-get install libfreetype6 libfreetype6-dev libfontconfig libfontconfig-dev`
+  * libpng
+    * suggested Ubuntu packages: libpng12-0, libpng12-dev e.g., `sudo apt-get install libpng12-0 libpng12-dev`
+  * libgif
+    * suggested Ubuntu package: libgif-dev e.g., `sudo apt-get install libgif-dev`
+    * `$ sudo apt-get install libgif4:i386`
+    * `$ sudo ln -s /usr/lib/i386-linux-gnu/libgif.so.4 /usr/lib/i386-linux-gnu/libgif.so`
+  * libglu1-mesa-dev
+  * mesa-common-dev
+  * GL
+    * such as freeglut3-dev
+  * Poppler PDF rendering library C++ development files
+    * suggested Ubuntu package: libpoppler-cpp-dev
+
+Check out the source code
+-------------------------
+
+Follow the instructions [here](../download) for downloading the Skia source.
+
+
+Generate build files
+--------------------
+
+We use the open-source gyp tool to generate ninja files (and analogous build
+scripts on other platforms) from our multiplatform "gyp" files.
+
+Generate the build files by running the following in your Skia home directory:
+
+    ./gyp_skia
+
+Or, you can just rely on it being run automatically by using `make` instead of
+`ninja` in examples shown below.
+
+If you want to use Eclipse, see Creating an Eclipse Project after you have generated the makefiles.
+
+Build and run tests from the command line
+-----------------------------------------
+
+    ninja -C out/Debug dm
+    out/Debug/dm
+
+The usual mode you want for testing is Debug mode (`SK_DEBUG` is defined, and
+debug symbols are included in the binary). If you would like to build the
+
+Release version instead
+-----------------------
+
+    ninja -C out/Release dm
+    out/Release/dm
+
+Build and run nanobench (performance tests)
+-------------------------------------------
+
+In this case, we will build with the "Release" configuration, since we are
+running performance tests.
+
+    ninja -C out/Release nanobench
+    out/Release/nanobench [ --skps path/to/*.skp ]
+
+Build and run SampleApp
+-----------------------
+
+This time we will add the `-j` flag to fire up multiple threads during the
+build. (This can be used with the other targets too.)
+
+    make -j SampleApp
+    out/Debug/SampleApp
+
+When this launches, you should see a window with various graphical examples.
+To move through the sample app, use the following keypresses:
+
+  * right-arrow key: cycle through different test pages
+  * left-arrow key: cycle through rendering methods for each test page
+  * other keys are defined in SampleApp.cpp’s SampleWindow::onHandleKey() and SampleWindow::onHandleChar() methods
+
+Build and run gm ("golden master") tests
+----------------------------------------
+
+This will display the return value (0 = success) after running the tests...
+
+    make -j gm
+    out/Debug/gm -r gm/base-linux ; echo $?
+
+You can also adjust the type used to represent SkScalar. By default, we use a
+float. To change that, run it as follows:
+
+    GYP_DEFINES="skia_scalar=fixed" make -j gm
+    out/Debug/gm -r gm/base-linux-fixed ; echo $?
+
+Build and run bench (performance testbench)
+-------------------------------------------
+
+Since bench tests performance, it usually makes more sense to run it in
+Release mode...
+
+    make -j bench BUILDTYPE=Release
+    out/Release/bench
+
+Build tools
+-----------
+
+    make -j tools
+    out/Debug/skdiff
+
+Clean up all generated files
+----------------------------
+
+    make clean