Change blimp README to use Markdown.

Going forward, the README.md and linked documents will be the
main source of developer documentation. The top-level
README.md will link to sections focused on items like building,
testing, etc.

BUG=None

Review URL: https://codereview.chromium.org/1353593002

Cr-Commit-Position: refs/heads/master@{#349935}

1 files changed, 50 insertions, 0 deletions

diff --git a/blimp/docs/build.md b/blimp/docs/build.md
new file mode 100644
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+# Using GN
+Blimp only supports building using [GN](../../tools/gn/README.md), and only
+supports building for Android and Linux. A quick overview over how to use GN can
+be found in the GN [quick start guide](../../tools/gn/docs/quick_start.md).
+
+## Android setup
+To setup GN, run the following command:
+```
+gn args out-android/Debug
+```
+This will bring up an editor, where you can type in the following:
+
+```
+target_os = "android"
+is_debug = true
+is_clang = true
+is_component_build = true
+symbol_level = 1  # Use -g1 instead of -g2
+use_goma = true
+```
+
+## Linux setup
+For building for Linux, you can have a side-by-side out-directory:
+```
+gn args out-linux/Debug
+```
+Use the same arguments as above, but remove `target_os`.
+```
+is_debug = true
+is_clang = true
+is_component_build = true
+symbol_level = 1  # Use -g1 instead of -g2
+use_goma = true
+```
+
+# Building
+
+To build blimp, build the target ```blimp```.
+
+## Building for Android
+
+```
+ninja -C out-android/Debug blimp
+```
+
+## Building for Linux
+
+```
+ninja -C out-linux/Debug blimp
+```