Add code style documentation

docs/code-style.md

@@ -1,6 +1,136 @@
 # Code Style
+The intent here is to maintain a common style across the project
+and rely on the process to enforce it instead of individuals.
 
-The [pom.xml](pom.xml) is configured to enforce a coding style defined in [checkstyle.xml](checkstyle.xml).
+## Automated code style check 
+The [pom.xml](../pom.xml) is configured to enforce a coding style 
+defined in [checkstyle.xml](../checkstyle.xml) when maven `validate` phase executed.
 
-The intent here is to maintain a common style across the project 
-and rely on the process to enforce it instead of individuals.
+## Formatting
+The project uses formatting rules described in `.editorconfig` file. Most of the popular IDEs have support of it.
+For example, in `IntelliJ IDEA` hit `Code -> Reformat Code` to organize your code.
+
+## Code conventions
+
+### Max line length
+Line length is limited to 120 columns.
+
+### Transitive dependencies
+Don't use transitive dependencies in project code.
+If it needed, recommended adding a library as a dependency of Maven in `pom.xml` directly.
+
+### Avoid wildcards in imports
+Do not use wildcard in imports because they hide what exactly is required by the class.
+```
+// bad
+import java.util.*;
+
+
+// good
+import java.util.HashMap;
+import java.util.Map;
+```
+
+### Variable and method naming
+Preferred to use `camel-case` naming convention for variables and methods.
+
+Name of variable should be self-explanatory:
+```
+// bad
+String s = resolveParamA();
+
+// good
+String resolvedParamA = resolveParamA();
+```
+This helps to other developers flesh your code out better without additional questions.
+
+For `Map`s it is recommended to use `To` between key and value designation:
+```
+// bad
+Map<Imp, ExtImp> map = getData();
+
+// good
+Map<Imp, ExtImp> impToExt = getData();
+```
+
+### Make variables final
+It is recommended to declare variable as `final`- not strict but rather project convention to keep the code safe.
+```
+// bad
+String value = "value";
+
+// good
+final String value = "value";
+```
+
+### Ternary operators
+Results of long ternary operators should be on separate lines:
+```
+// bad
+boolean result = someVeryVeryLongConditionThatForcesLineWrap ? firstResult
+    : secondResult;
+
+// good
+boolean result = someVeryVeryLongConditionThatForcesLineWrap
+    ? firstResult
+    : secondResult;
+```
+
+No so strict, but short ternary operations should be in one line:
+```
+// bad
+boolean result = someShortCondition
+    ? firstResult
+    : secondResult;
+
+// good
+boolean result = someShortCondition ? firstResult : secondResult;
+```
+
+### Nested method calls
+Try to avoid hard-readable multiple nested method calls:
+```
+// bad
+int resolvedValue = resolveValue(fetchExternalJson(url, httpClient), populateAdditionalKeys(mainKeys, keyResolver));
+
+// good
+String externalJson = fetchExternalJson(url, httpClient);
+List<Key> additionalKeys = fetchAdditionalKeys(mainKeys, keyResolver);
+int resolvedValue = resolveValue(externalJson, additionalKeys);
+```
+
+### Check for NULL
+If you're dealing with incoming data, please ensure to check if nested object is not null before chaining.
+```
+// bad
+final ExtRequestTargeting targeting = bidRequest.getExt().getPrebid().getTargeting();
+
+// good
+final ExtRequest requestExt = bidRequest.getExt();
+final ExtRequestPrebid prebid = requestExt != null ? requestExt.getPrebid() : null;
+final ExtRequestTargeting targeting = prebid != null ? prebid.getTargeting() : null;
+```
+For convenience, the `org.prebid.server.util.ObjectUtils` helper can be used for such kind of operations.
+
+### Garbage code
+Don't leave commented code (don't think about the future). You can always add it later when it will be really desired. 
+```
+// bad
+// String iWillUseThisLater = "never";
+```
+
+### Privacy
+It is strictly prohibited to log any of private data about publisher, exchanges or similar sensitive information.
+The idea is to keep this open-source project safe as far as possible.
+
+### Tests
+The code should be covered over 90%.
+
+Don't use real data in tests, like endpoint URLs, account IDs, etc.
+```
+// bad
+String ENDPOINT_URL = "https://prebid.org";
+
+// good
+String ENDPOINT_URL = "https://test-endpoint.url";
+```