CODING_STYLE: specify the indent rule for multiline code

We didn't specify the indent rule for multiline code here, which may
mislead users. And in current code, the code use various styles.

Add this rule in CODING_STYLE to make sure this is clear to every one.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Suggested-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190304071631.27567-2-richardw.yang@linux.intel.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

CODING_STYLE

@@ -29,6 +29,45 @@ Spaces of course are superior to tabs because:
 
 Do not leave whitespace dangling off the ends of lines.
 
+1.1 Multiline Indent
+
+There are several places where indent is necessary:
+
+ - if/else
+ - while/for
+ - function definition & call
+
+When breaking up a long line to fit within line width, we need a proper indent
+for the following lines.
+
+In case of if/else, while/for, align the secondary lines just after the
+opening parenthesis of the first.
+
+For example:
+
+    if (a == 1 &&
+        b == 2) {
+
+    while (a == 1 &&
+           b == 2) {
+
+In case of function, there are several variants:
+
+    * 4 spaces indent from the beginning
+    * align the secondary lines just after the opening parenthesis of the
+      first
+
+For example:
+
+    do_something(x, y,
+        z);
+
+    do_something(x, y,
+                 z);
+
+    do_something(x, do_another(y,
+                               z));
+
 2. Line width
 
 Lines should be 80 characters; try not to make them longer.