feat(ch06): add functional programming theory

src/ch-06_what-is-fp/README.md

@@ -0,0 +1,31 @@
+# What is Functional Programming?
+
+## Functional Programming
+- Programming paradigm
+- Apps are composed 
+    - Using Pure functions
+    - Avoiding shared mutable state
+    - Without side-effects
+- More declarative than imperative
+
+### Pure Functions
+- Same inputs always return the same output
+- No side-effects
+
+* Referential transparency, replace a function call with a value does not affect the programm.
+
+### Function Composition
+- Combine two or more functions into one
+```
+f(g(x))
+```
+
+### Shared State
+- Race condition
+- Different results depending on the order in which functions are called
+
+* By avoiding shared state, timing and order of function calls do not affect the result
+
+
+
+