Add extension function doc and update README to add link to extension doc

Author: enginebai

EXTENSIONS.md

@@ -0,0 +1,54 @@
+# Useful Extension Functions
+## View
+```kotlin
+// Prevent multiple/duplicate click
+View.debounceClick() { ... } 
+
+// Visibility
+View.visible()
+View.invisible()
+View.gone()
+View.showIf { ... }
+View.hideIf { ... }
+View.goneIf { ... }
+
+// Keyboard
+View.showKeyboard()
+View.hideKeyboard()
+
+// Dimension
+View.px2dp()
+View.dp2px()
+View.px2sp()
+View.sp2px()
+
+// Resources
+View.getColor(R.color.colorPrimary)
+View.getDrawable(R.drawable.ic_launcher)
+```
+
+## EditText
+```kotlin
+EditText.textChanged { s -> ... }
+EditText.textChanged().doOnNext { s -> ... }.subscribe()
+EditText.validate(errorMessage = "The name should not be empty") { !it.isNullOrBlank() }
+EditText.validateEmail { ... }
+EditText.validateEmail("It's not valid email")
+EditText.showPassword()
+EditText.hidePassword()
+```
+
+## TextView
+```kotlin
+// Set view gone while text is null or empty.
+TextView.setTextWithExistence("1234")
+
+// Set view invislble while text is null or empty.
+TextView.setTextWithVisibility("5678")
+```
+
+## String
+```kotlin
+String?.isValidEmail(): Boolean
+```
+

README.md

@@ -147,6 +147,9 @@ fun Project.importCommonDependencies() {
 * [Timber](https://github.com/JakeWharton/timber), for logging.
 * [Epoxy](https://github.com/airbnb/epoxy), for RecyclerView complex view layout.
 
+## Useful Extensions
+* See [Extension Functions.](./EXTENSIONS.md)
+
 ## How to Update
 Keep this repository as one of your project tracked remote.