added a coding style section

CONTRIBUTING.md

@@ -1,17 +1,173 @@
 # Contribution guidelines
 
-#### Always work on feature branches
+## Coding style
+
+### Naming
+
+Variables should use lower case names with underscores.
+
+There are now special visibility or type prefixes.
+
+Structure members should not use abbreviated names:
+
+Good
+```
+struct A {
+    char *message;
+    int length;
+}
+```
+
+Bad:
+```
+struct A {
+    char *msg;
+    int len;
+}
+```
+
+### Placement of spaces
+
+In variable declarations (including in function parameters) we place the asterisk next to the variable name.
+
+Good:
+```c
+int *number;
+```
+
+Bad:
+```c
+int * number;
+int* number;
+```
+
+In function return values we place the asterisk between spaces
+
+Good:
+```c
+void * my_thread() {
+    
+}
+```
+
+Bad:
+```c
+void *mythread() {
+    
+}
+
+void * mythread() {
+    
+}
+```
+
+In expressions we place spaces between most elements but not between parantheses:
+
+Good:
+```
+int a = ((5 * 10) + 10 ) % 22;
+```
+
+Bad:
+```
+int a = ((5*10)+10)%22;
+```
+
+We also put spaces infront / after most keywords such as `if`, `while`, ...
+
+Good:
+```c
+for (int a = 0; a < 1; a++) {
+    ...
+}
+```
+
+Bad:
+```c
+for(int a = 0; a < 1; a++) {
+    ...
+}
+```
+
+### Line length
+
+Try to limit line length to about 160 characters.
+
+### Indention
+
+We use tabs for indention which should be displayed as 8 spaces by your editor.
+
+### Brace placement
+
+For everything except functions we place the opening brace in the same line:
+
+Good:
+```c
+struct A {
+    enum {
+        A,
+        B
+    } choice;
+}
+
+if (true) {
+    
+}
+```
+
+Bad:
+```
+struct A
+{
+    enum
+    {
+        A,
+        B
+    } choice;
+}
+
+if (true)
+{
+    
+}
+```
+
+However, in functions we place the brace in a new line:
+
+```
+void main()
+{
+    
+}
+```
+
+### #define and Macro Names
+
+Are always written UPPERCASE.
+
+Good:
+```c
+#define HUGEPAGESIZE (1 << 22)
+#define MAX(a, b) (a > b ? a ? b)
+```
+
+Bad:
+```c
+#define max(a, b) (a > b ? a ? b)
+```
+
+## Always work on feature branches
 
 Please branch from `develop` to create a new _feature_ branch.
 
 Please create a new _feature_ branch for every new feature or fix.
 
-#### Do not commit directly to `master` or `develop`.
+## Do not commit directly to `master` or `develop`.
 
 Use your _feature_ branch.
 
 Please rebase your work against the `develop` before submitting a merge reqeuest.
 
-#### Make the CI happy :-)
+## Make the CI happy :-)
 
 Only branches which pass the CI can be merged.