docs: add sections to development tips in creating blocks

docs/3.creating-blocks.md

@@ -1308,13 +1308,17 @@ if a future version of LuaJIT implements the Lua 5.3 language, which has a
 
 ## Development Tips
 
+#### REPL
+
 For prototyping with Lua, the LuaJIT REPL is a bit too minimalist to be
 productive. As in the original Lua 5.1 REPL, it requires preceding expressions
 with "=" to print their value, and it doesn't support dumping tables. I suggest
 using Torch's [trepl](https://github.com/torch/trepl) for prototyping,
 available in [LuaRocks](https://luarocks.org/modules/vsergeev/trepl-torchless).
 It supports table dumping, pretty printing, tab completion, and line history.
 
+#### Prototyping
+
 For some blocks, it may be more productive to prototype signal processing
 routines in Python, with libraries like [numpy](http://www.numpy.org/),
 [scipy](http://docs.scipy.org/doc/scipy/reference/index.html), and