Update build instructions. Add "Building old unsupported version

cztomczak · cztomczak · commit ad7e5b79f2f3 · 2018-02-06T15:45:10.000+01:00
of Chromium" section.
diff --git a/docs/Build-instructions.md b/docs/Build-instructions.md
@@ -12,6 +12,7 @@ Table of contents:
 * [Build using prebuilt CEF binaries and libraries](#build-using-prebuilt-cef-binaries-and-libraries)
 * [Build using CEF binaries from Spotify Automated Builds](#build-using-cef-binaries-from-spotify-automated-builds)
 * [Build upstream CEF from sources](#build-upstream-cef-from-sources)
+  * [Building old unsupported version of Chromium](#building-old-unsupported-version-of-chromium)
   * [Possible errors](#possible-errors)
 * [Build CEF manually](#build-cef-manually)
 * [CEF Automated Builds (Spotify and Adobe)](#cef-automated-builds-spotify-and-adobe)
@@ -300,6 +301,7 @@ python ../tools/build.py xx.x
 
 ## Build upstream CEF from sources
 
+
 Building CEF from sources is a very long process that can take several
 hours depending on your CPU speed and the platform you're building on.
 To speed up the process you can pass the --fast-build flag, however
@@ -351,6 +353,26 @@ module, make installer package, install the package and run unit tests
 and examples. See the notes for commands for creating package installer
 and/or wheel package for distribution.
 
+### Building old unsupported version of Chromium 
+
+When building an old version of Chromium you may get into issues.
+For example as of this writing the latest CEF Python version is
+v57, but current support Chromium version is v64. Now when building
+v57 you may encounter issues since Chromium build tools had
+many updates since v57. You have to checkout depot_tools from the
+revision when Chromium v57 was released. When running automate.py
+tool the depot_tools repository resides in `build_dir/depot_tools/`
+directory. If you didn't run automate.py then you can find repository
+url in automate-git.py script. For example for v57 release to checkout
+an old revision of depot_tools you can use this command:
+
+```
+git checkout master@{2017-04-20}
+```
+
+After that set `DEPOT_TOOLS_UPDATE=0` environment variable and then
+run automate.py tool.
+
 ### Possible errors
 
 __Debug_GN_arm/ configuration error (Linux)__: Even though building
