Add simple doc for build.

Signed-off-by: Eric Wang <[email protected]>

docs/build.md

@@ -0,0 +1,38 @@
+# How to build
+
+Be careful with the versions of the build tools (see below) you installed on your box, usually the latest `ionic` and `cordova` might have unexpected bugs...
+
+
+|Tool   |Version|Install|
+|-------|-------|-------|
+|nodejs |latest |check this [guide](https://skygragon.github.io/leetcode-cli/install) for help|
+|ionic  |3.19.1 |npm install -g [email protected]|
+|cordova|7.1.0  |npm install -g [email protected]|
+
+After installed, check followings before going forward.
+
+```
+$ node -v
+$ npm -v
+$ ioinc info
+```
+
+## Android
+
+|Tool   |Version|Install|
+|-------|-------|-------|
+|Android SDK Tools|24.4.1|[Download](http://www.androiddevtools.cn/)|
+|Android SDK|4.0 (android-14)|Use `sdkmanager` in Android SDK tools|
+
+Build the apk:
+
+```
+$ git clone http://github.com/skygragon/shlib
+$ cd shlib
+$ ionic cordova platform add android
+$ ionic cordova build android
+```
+
+## IOS
+
+TBD