add "how to build for ios" to readme

engine/README.md

 ```
 
 
+## How to Build (IOS)
+
+The UIWidgets engine for iOS can only be built on Mac.
+
+### Preliminaries
+
+There is a special settings, namely "Enable bitcode" we need set properly when building an ios native plugin.
+Generally, the program is possible to run faster when "Enable bitcode" is true while the size of the plugin
+will also become bigger.
+
+You can choose to build the library with enabled or disabled "Enable bitcode" using different build commands as 
+shown in the following two sections.
+
+You can also build UIWidgets engine with enabled or disabled "Enable bitcode" by changing the value of 
+"ios_bitcode_enabled" in "Build.bee.cs". Note that the value is set to false by default.
+
+You can also change the setting of "Enable bitcode" in the XCode project generated by Unity. Specifically, you can 
+find this setting in "Build Settings/Build Options/Enable Bitcode". This value is true by default.
+
+You should always keep the "Enable bitcode" settings the same in the built library, the UIWidgets engine and the XCode project generated by your UIWidget project.
+
+### Build Dependencies
+
+Setting up the Engine development environment
+
+Please follow https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment.
+
+Check out repo and update dependencies:
+
+```
+cd $FLUTTER_ROOT/flutter
+git checkout flutter-1.17-candidate.5
+gclient sync -D
+```
+
+Apply following to end of `flutter/third_party/txt/BUILD.gn`
+```
+diff --git a/third_party/txt/BUILD.gn b/third_party/txt/BUILD.gn
+index 56b73a020..d42e88045 100644
+--- a/third_party/txt/BUILD.gn
++++ b/third_party/txt/BUILD.gn
+@@ -141,6 +141,7 @@ source_set("txt") {
+     "//third_party/harfbuzz",
+     "//third_party/icu",
+     "//third_party/skia",
++    "//third_party/skia/modules/skottie",
+   ]
+ 
+   deps = [
+@@ -339,3 +340,10 @@ executable("txt_benchmarks") {
+     deps += [ "//third_party/skia/modules/skparagraph" ]
+   }
+ }
++
++static_library("txt_lib") {
++  complete_static_lib = true
++  deps = [
++    ":txt",
++  ]
++}
+```
+
+Comiple engine:
+```
+cd $FLUTTER_ROOT
+./flutter/tools/gn --unoptimized --ios            //disable bitcode
+./flutter/tools/gn --unoptimized --ios --bitcode  //enable bitcode
+ninja -C out/ios_debug_unopt/ flutter/third_party/txt:txt_lib
+```
+
+If the compilation fails because "no available Mac SDK is found" (in flutter-1.17 the build tool will only try to find Mac 10.XX SDKs), please modify the file "/src/build/Mac/find_sdk.py" under flutter root by setting "sdks" as your current sdk, e.g., ['11.0']. 
+
+If the compilation fails because "'Foundation/NSURLHandle.h' file not found" (flutter-1.17 assumes that you are using some low iphone SDK (e.g., older than 12.2), in which some platform-dependent Macros are defined differently from new SDKs like 12.2), please modify the file "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/TargetConditionals.h" in your system by making "TARGET_OS_EMBEDDED = 1" and "TARGET_OS_MACCATALYST = 0" for arm64-iphone architecture. You can also work-around this issue by checking out a new version of flutter (e.g., "flutter-1.18-candidate.6") and run "gclient sync -D" to get dependencies on newer iphone SDKs. Then switch back and build.
+
+### Creat symbolic
+
+Create symbolic as follows. Flutter engine txt include skia header in this pattern 'third_party/skia/*', so without symbolic, the txt lib will include skia
+header file in flutter engine, instead of headers in skia repo.
+
+cmd
+```
+cd <uiwidigets_dir>\engine
+cd third_party   \\ create the directory if not exists
+ln -s <SKIA_ROOT> skia
+```
+
+### Build Engine
+
+```
+cd <uiwidigets_dir>\engine
+mono bee.exe ios
+```