Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -6,10 +6,10 @@ Contributing to Zeppelin
 These are some global design goals in Zeppelin.
 
 ### D0 - Security in Depth
-We strive to provide secure, tested, audited code. To achieve this, we need to match intention with function. Thus, documentation, code clarity and community review and security discussions are fundamental.
+We strive to provide secure, tested, audited code. To achieve this, we need to match intention with function. Thus, documentation, code clarity, community review and security discussions are fundamental.
 
 ### D1 - Simple and Modular
-Simpler code means easier audits, and better understanding of what each component does. We look for small files, small contracts, and small functions. If you can separate a contract into two independent functionalities you probably should do it.
+Simpler code means easier audits, and better understanding of what each component does. We look for small files, small contracts, and small functions. If you can separate a contract into two independent functionalities you should probably do it.
 
 ### D2 - Naming Matters
 
@@ -17,14 +17,18 @@ We take our time with picking names. Code is going to be written once, and read
 
 ### D3 - Tests
 
-Write a test for all your code. We encourage Test Driven Development so we know when our code is right. 
+Write tests for all your code. We encourage Test Driven Development so we know when our code is right. Even though not all code in the repository is tested at the moment, we aim to test every line of code in the future.
 
 ### D4 - Check preconditions and post-conditions
-TODO
 
-### D5 - Consistency Everywhere
+A very important way to prevent vulnerabilities is to catch a contract’s inconsistent state as early as possible. This is why we want functions to check pre- and post-conditions for executing its logic. When writing code, ask yourself what you are expecting to be true before and after the function runs, and express it in code.
+
+### D5 - Code Consistency
+
+Consistency on the way classes are used is paramount to an easier understanding of the library. The codebase should be as unified as possible. Read existing code and get inspired before you write your own. Follow the style guidelines. Don’t hesitate to ask for help on how to best write a specific piece of code.
 
-Consistency on the way classes are used is paramount to allow an easier understanding of the library.
+### D6 - Regular Audits
+Following good programming practices is a way to reduce the risk of vulnerabilities, but professional code audits are still needed. We will perform regular code audits on major releases, and hire security professionals to provide independent review.
 
 ## Style Guidelines