add new brace guidelines and clang-format info (#3414)

Add new brace guidelines, correct code sample.
Add detail to indentation guidelines.
Add basic clang-format information and a link to the documentation on llvm.

closes #3413 
A quick edit to the contributing guide from the web ui, hope it comes out right.

.github/CONTRIBUTING.md

@@ -91,35 +91,28 @@ If you find any usage of the old keywords, we encourage you to fix it.
 
 ### Braces ###
 
-Braces should almost always go on their own line:
+Braces should go on their own line except for control statements, the use of braces around single line statements is preferred.
+See the following example:
 ```c++
 int main()
-{
-    if (someCondition)
-    {
-        doSomething();
-    }
-    else if (someOtherCondition1)
-    {
-        for (int i = 0; i < 100; i++)
-        {
+{ // function or class: own line
+    if (someCondition) { // control statement: same line
+        doSomething(); // single line statement, braces preferred
+    } else if (someOtherCondition1) { // else goes after closing brace
+        for (int i = 0; i < 100; i++) {
             doSomethingElse();
         }
-    }
-    else
-    {
-        while (someOtherCondition2)
-        {
+    } else {
+        while (someOtherCondition2) {
             doSomethingElse();
         }
     }
 }
 ```
-Braces should never be omitted for single-statement. Keeping the code legibile is a high priority of ours and we hope you share a similar belief :)
 
-### Tabs vs Spaces ###
+### Indentation ###
 
-We _highly_ encourate the use of spaces. If you use tabs, please readjust them to 4 spaces per tab before submitting.
+Always indent using 4 spaces, do not use tabs. Opening and closing braces should be on the same indentation layer, member access specifiers in classes or structs should not be indented.
 
 ### Lines ###
 
@@ -127,6 +120,11 @@ Do not have trailing whitespace in your lines, if possible. Most IDEs check for
 
 Lines should be 120 characters or less, but you can exceed this if you find it necessary.
 
+### Automatic Formatting ###
+
+The handy tool `clang-format` can format your code for you, a special `.clang-format` configuration file is included [here](https://github.com/Cockatrice/Cockatrice/blob/master/.clang-format).
+See [the clang-format documentation](https://clang.llvm.org/docs/ClangFormat.html) for more information.
+
 ### Memory Management ###
 
 New code should be written using references over pointers and stack allocation