Add Attributes style guide.

Author: Crell

spec.md

@@ -1276,6 +1276,87 @@ function allowed()
         compliant
         nowdoc
         COMPLIANT;
+```
+
+## 11. Attributes
+
+### 12.1 Basics
+
+Attribute names must immediately follow the opening attribute block indicator `#[` with no space.
+
+If an attribute has no arguments, the `()` MUST be omitted.
+
+The closing attribute block indicator `]` MUST follow the last character of the attribute name or the closing `)` of
+its argument list, with no preceding space.
+
+### 12.2 Placement
+
+Attributes on classes, methods, functions, constants and properties MUST
+be placed on their own line, immediately prior to the structure being described.
+
+For attributes on parameters, if the parameter list is presented on a single line,
+the attribute MUST be placed inline with the parameter it describes, separated by a single space.
+If the parameter list is split into multiple lines for any reason, the attribute MUST be placed on
+its own line prior to the parameter, indented the same as the parameter.
+
+If a comment docblock is present on a structure that also includes an attribute, the comment block MUST
+come first, followed by any attributes, followed by the structure itself.  There MUST NOT be any blank lines
+between the docblock and attributes, or the attributes and the structure.
+
+If two separate attribute blocks (denoted by separate `#[]` markers) are used in a multi-line context,
+they MUST be on separate lines with no blank lines between them.
+
+### 12.3 Compound attributes
+
+Multiple attributes MAY be placed in the same attribute block (`#[]`) if and only if the entire block is listed on a
+single line.  They must be separated by a comma with a space following but no space preceding.  If the attribute list
+is split into multiple lines for any reason, then the attributes MUST be placed  in separate attribute blocks.
+Those blocks may themselves contain multiple attributes provided this rule is respected.
+
+If an attribute's argument list is split into multiple lines for any reason, then:
+
+* The attribute MUST be the only one in its attribute block.
+* The attribute arguments MUST follow the same rules as defined for multiline function calls.
+
+### 12.4 Example
+
+The following is an example of valid attribute usage.
+
+```php
+#[Foo]
+#[Bar('baz')]
+class Demo
+{
+    #[Beep]
+    private Foo $foo;
+
+    /**
+     * Sets the foo.
+     */
+     #[Poink('narf'), Narf('poink')]
+    public function setFoo(#[Beep] Foo $new): void
+    {
+      // ...
+    }
+
+    #[Complex(
+        prop: 'val',
+        other: 5,
+    )]
+    #[Other, Stuff, Here]
+    public function complicated(
+        string $a,
+        #[Decl]
+        string $b,
+        #[Complex(
+            prop: 'val',
+            other: 5,
+        )]
+        string $c,
+        int $d,
+    ): string {
+        // ...
+    }
 }
 ```