Add Developer's guide and best practices (#1075)

* Add Developer's guide and best practices

* Fix comments

CODING_GUIDELINES.md

@@ -0,0 +1,51 @@
+# Coding Guidelines
+
+This document is an extension to [CONTRIBUTING](./CONTRIBUTING.md) and provides more details about the coding guidelines and requirements.
+
+## API & Design
+
+* Code must be well structured:
+    * packages must have a limited responsibility (different concerns can go to different packages),
+    * types must be easy to compose,
+    * think about maintainbility and testability.
+* "Depend upon abstractions, [not] concretions".
+* Try to limit the number of methods you are exposing. It's easier to expose something later than to hide it.
+* Follow agreed-upon design patterns and naming conventions.
+* publicly-exposed functions are named logically, have forward-thinking arguments and return types.
+* Avoid global variables and global configurators.
+* Favor composable and extensible designs.
+* Minimize code duplication.
+* Limit third-party dependencies.
+
+Performance:
+
+* Avoid unnecessary operations or memory allocations.
+
+Security:
+
+* Pay proper attention to exploits involving:
+    * gas usage
+    * transaction verification and signatures
+    * malleability
+    * code must be always deterministic
+* Thread safety. If some functionality is not thread-safe, or uses something that is not thread-safe, then clearly indicate the risk on each level.
+
+## Best practices
+
+* Use [goimports](https://pkg.go.dev/golang.org/x/tools/cmd/goimports) as your code formatter.
+
+* Always wrap returned errors. 
+    * Doing `if err != nil { return err }` does not include each callers' context. Pushing errors up the stack without context makes it harder to test and debug. Additionally, a short context description makes it easier for the reader to understand the code. Example:
+  
+        ```go
+        if !coins.IsZero() { 
+            if err := k.bank.TransferCoins(ctx, caller, contractAddress, coins); err != nil { 
+                return nil, err 
+            } 
+        } 
+        ```
+
+    * It would be an improvement to return  `return nil, sdkerror.Wrap(err, "lock contract coins")`
+    * Please notice that fmt.Errorf is not used, because the error handling predates fmt.Errorf and errors.Is 
+   
+* Limit the use of aliases, when not used during the refactoring process.