add documentation for let and while forms

Readme.org

@@ -69,9 +69,38 @@ Function calls are executed as soon as the tree is evaluated. See the following
 
 In this example, ~(add 5 2)~ is evaluated first, its result is then passed to ~(add 3 ...)~. In infix form: ~3 + (5 + 2)~.
 
-*** TODO Control flow
-**** TODO if
-**** TODO while
+*** Control flow
+**** if
+An if form is the most basic form of conditional evaluation offered by Relish.
+It is a function that takes lazily evaluated arguments: a condition, a then clause, and an else clause.
+If the condition evaluates to true, the then clause is evaluated and the result returned.
+Otherwise the else clause is evaluated and the result is returned.
+If the condition evaluates to neither true nor false (a non-boolean value) a type error is returned.
+
+#+BEGIN_SRC lisp
+  ;; simple condition
+  (if true
+      (echo "its true!")
+      (echo "its false!"))
+
+  ;; more advanced condition, with hypothetical data
+  (if (get-my-flag global-state)
+      (echo "my flag is already on!")
+      (turn-on-my-flag global-state))
+#+END_SRC
+
+**** while
+Another popular control flow structure is the *while loop*.
+This is implemented as a condition followed by one or more bodies that are lazily evaluated only if the condition is true.
+Like the *if form*, if the conditional returns a non-boolean value the *while loop* will return an error.
+
+#+BEGIN_SRC lisp
+  (while (get-my-flag global-state) ;; if false, returns (nothing) immediately
+    (dothing) ;; this is evaluated
+    "simple token" ;; this is also evaluated
+    (toggle-my-flag global-state)) ;; this is also evaluated
+#+END_SRC
+
 **** TODO let
 **** TODO circuit
 *** TODO quote and eval
@@ -166,7 +195,13 @@ Errors during configuration are non-terminal. In such a case any defaults which
 - CFG_RELISH_PROMPT (default (echo "λ ")): A *function* definition which is called in order to output the prompt for each loop of the REPL.
   This function will be reloaded each REPL loop and will be called by the interpreter with no arguments.
 
-** TODO Further configuration
+*** Further configuration
+Further configuration can be done by loading scripts that contain more functions and data to evaluate.
+Variables and functions defined in an external script loaded by your interpreter will persist in the symbol table.
+
+#+BEGIN_SRC lisp
+  (load "my-extra-library-functions.rls")
+#+END_SRC
 
 ** Compilation 
 #+BEGIN_SRC sh