more to dev guide

DEV_NOTE2.md

@@ -203,4 +203,10 @@ so that it can be used in an LPhy script.
 In LPhy core, the registration is normally in the class [lphy.base.spi.LPhyBaseImpl](https://github.com/LinguaPhylo/linguaPhylo/blob/214bb91e8546fc136d51fb50ab6555522a8385a6/lphy-base/src/main/java/lphy/base/spi/LPhyBaseImpl.java#L46-L120). 
 Add your class into the corresponding list.
 
+To add and register new `SequenceType` in LPhy extension, you need to:
 
+1. create your own class and inherit (extends) `SequenceTypeBaseImpl` ;
+2. initialize Map using the same code in the constructor;
+3. copy the method `register()`;
+4. overwrite `declareSequenceTypes()` and register your `SequenceType` there;
+5. overwrite `getExtensionName()`.

lphy-base/src/main/java/lphy/base/spi/SequenceTypeBaseImpl.java

@@ -30,8 +30,13 @@ public class SequenceTypeBaseImpl implements SequenceTypeExtension {
         return dataTypeMap;
     }
 
-    /**TODO private or protected?
-     * LPhy sequence types {@link SequenceType}
+    /**
+     * To add and register new SequenceType in LPhy extension, you need to:
+     * 1. create your own class and inherit (extends) SequenceTypeBaseImpl ;
+     * 2. copy the constructor to init Map;
+     * 3. copy the method {@link #register()};
+     * 4. overwrite {@link #declareSequenceTypes()} and register your SequenceType there;
+     * 5. overwrite {@link #getExtensionName()}.
      */
     protected static Map<String, SequenceType> dataTypeMap;