Add Kotlin Coding Guideline

doc/kotlin-coding-guideline.md

@@ -0,0 +1,27 @@
+### `value.?let` or `if (value != null)`
+
+Is really better to use
+
+```kotlin
+value?.let {
+     // some block
+}
+```
+
+then
+
+```kotlin
+if (value != null) {
+    // some block
+}
+```
+
+There is no return value so block is NOT expression.
+
+### Don't use anonymous class implementation
+```kotlin
+val value = object : Type {
+    // implementation
+}
+```
+