Add quic transport doc

Author: qwu16

quic_transport/docs/build_instructions.md

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+# Building OWT QUIC SDK
+
+## System requirements
+
+- At least 50GB of free disk space.
+- High speed network connection.
+- Ubuntu 18.04 for Ubuntu build.
+- Chromium source code base 109.0.5393.1
+
+## Install dependencies
+
+Please follow [Chromium Windows build instruction](https://chromium.googlesource.com/chromium/src/+/HEAD/docs/windows_build_instructions.md) or [Chromium Linux build instruction](https://chromium.googlesource.com/chromium/src/+/HEAD/docs/linux/build_instructions.md) to setup system and install `depot_tools`.
+
+## Get the code
+
+Create a new directory for the check out, and create a `.gclient` file in this directory. Add following code to `.gclient` file.
+
+```
+solutions = [
+  { "name"        : "src/owt",
+    "url"         : "https://github.com/open-webrtc-toolkit/owt-sdk-quic.git",
+    "deps_file"   : "DEPS",
+    "managed"     : False,
+    "custom_deps" : {
+    },
+    "custom_vars": {},
+  },
+]
+```
+
+Run `gclient sync` to check out SDK code, Chromium code, and other dependencies. It may take one or two hours if your network connection is not fast enough.
+
+You will see a `src` directory after sync completes. Switch to the `src` directory for following steps.
+
+## Additional changes
+
+Some manually changes to Chromium code are needed before building SDK.
+
+1. Apply patches in `owt/quic_transport/patches` to `src` directory.
+
+1. Create a file `gclient_args.gni` in `build/config` with following code.
+
+```
+# Generated from 'DEPS'
+build_with_chromium = true
+checkout_android = false
+checkout_android_native_support = false
+checkout_ios_webkit = false
+checkout_nacl = true
+checkout_oculus_sdk = false
+checkout_openxr = false
+checkout_aemu = false
+checkout_google_benchmark = false
+```
+
+Since we checked out code to `src/owt`, gclient cannot find buildtools under this directory. We need to add an environment variable `CHROMIUM_BUILDTOOLS_PATH`. Its value should be `<dir of .gclient file>/src/buildtools`.
+
+## Build SDK
+
+Run `gn gen out/debug` to generate ninja files, or `gn args out/debug` to configure GN arguments. For debug version, it may look like this
+```
+is_debug=true
+is_component_build=false
+symbol_level=1
+```
+
+You may want to set `is_component_build` to `false` in order to get a single shared library, but you can also set it to `true` to reduce the compiling time for debugging. `symbol_level` is set to `1` since `2` is conflicted with `is_component_build=false`.
+
+Then run `ninja -C out/debug/ owt_quic_transport` to build the SDK.
+
+## Certificates
+
+Encryption is mandatory for QUIC connections. You may generate a testing certificate by running `net/tools/quic/certs/generate-certs.sh`. It valids for 72 hours.