fix: README links, line breaks between methods

lib/index.js

@@ -30,6 +30,7 @@ let Equation = {
 
     return stack;
   },
+
   /**
     * Creates an equation function which replaces variables
     * in the given expression with the values specified in order,
@@ -72,6 +73,7 @@ let Equation = {
   registerOperator(key, options) {
     operators[key] = options;
   },
+
   registerConstant(key, options) {
     constants[key] = options;
   }