Added coding contribution.md to documentation (#847)

Part of https://github.com/radareorg/cutter/issues/832

docs/contributing.md

@@ -0,0 +1,82 @@
+# Contributing
+
+This page shows some hints about the coding conventions.
+**Disclaimer:** It is a work and progress and we will provide soon a fully documented API.
+
+## Coding advices
+
+### CutterCore class
+
+This is the main class where every link with r2 is made. It is *unique* accross the whole process. To access it, simply call `Core()`.
+
+Example:
+```c++
+Core()->getOffset();
+```
+
+### Calling a radare2 command
+
+There are two ways to do it:
+* `CutterCore::cmd()` *(Discouraged)* Only use it for commands which yells no output
+* `CutterCore::cmdj()` To be used with json commands like `cmdj("agj")` or `cmdj("aflj")`. It is way easier to parse a json output.
+
+Example:
+```c++
+QJsonArray array = Core()->cmdj("pdj 1 @ main").array();
+```
+
+### Seek the current file
+
+To modify radare2 seek use `CutterCore::seek(const RVA offset)`. This is important because it will emit a `CutterCore::seekChanged(RVA offset)` signal.
+Never ever call `cmd("s offset")`;
+
+Example:
+```c++
+Core()->seek(0xdeadbeef);
+```
+
+### Creating a widget
+
+Make sure to connect the `CutterCore::seekChanged(RVA offset)` signal so your widget refreshes its output when radare2 seek is modified (switching to another function, etc.).
+
+## General coding guidelines
+
+### Coding style
+
+We follow [these guidelines](https://wiki.qt.io/Qt_Coding_Style) to format the code.
+If in doubt, you can use [AStyle 2.06](https://sourceforge.net/projects/astyle/files/astyle/astyle%202.06/) to format the code. The command line for formatting the code according to the style is:
+
+```bash
+astyle --project=src/Cutter.astylerc src/filename.cpp
+```
+
+#### Loops
+
+We use C++11 foreach loop style which means any "foreach" loop should look like:
+```c++
+for (QJsonValue value : importsArray) {
+    doSomething(value);
+}
+```
+
+#### Nullptr
+
+Please do not use `0` nor `Q_NULLPTR`, only use `nullptr`.
+
+Example:
+```c++
+QObject *object = nullptr;
+```
+
+#### Connecting signals
+
+To connect a signal to a slot, this is the preferred way to do it:
+```c++
+connect(sender, &QObject::destroyed, this, &MyObject::objectDestroyed);
+```
+The main reason is that this syntax allows the use of lambda functions.
+
+### Functions documentation
+
+It's good to add some documentation to your functions when needed. To do so we follow these [rules](http://doc.qt.io/qt-5/qdoc-guide-writing.html).
+