[Contributing] [Conventions] Added entry for Yoda conditions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.8+
| Fixed tickets |

Author: phansys

contributing/code/conventions.rst

@@ -77,6 +77,31 @@ must be used instead (where ``XXX`` is the name of the related thing):
     "replaceXXX", on the other hand, cannot add new elements. If an unrecognized
     key is passed to "replaceXXX" it must throw an exception.
 
+.. _contributing-code-conventions-conditions:
+
+Conditions
+----------
+
+Use `Yoda condition`_ when you need to check a variable against a value.
+The main goal with this syntax is to avoid an accidental assignment inside the
+condition statements:
+
+instead of:
+
+.. code-block:: php
+
+    if ($someVar === 'some string' && $otherVar === someFunction()) {
+        // ...
+    }
+
+it should be:
+
+.. code-block:: php
+
+    if ('some string' === $someVar && someFunction() === $otherVar) {
+        // ...
+    }
+
 .. _contributing-code-conventions-deprecations:
 
 Deprecations
@@ -104,3 +129,5 @@ the migration starting one or two minor versions before the version where the
 feature will be removed (depending on the criticality of the removal)::
 
     trigger_error('XXX() is deprecated since version 2.X and will be removed in 2.Y. Use XXX instead.', E_USER_DEPRECATED);
+
+.. _`Yoda condition`: https://en.wikipedia.org/wiki/Yoda_conditions