Add naming rules doc

docs/naming-rules.md

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+# Naming rules
+
+Rules are:
+
+* Made of lowercase words separated by hyphens.
+* Split into two parts:
+  * The first describes [what thing](http://apps.workflower.fi/vocabs/css/en) the rule applies to.
+  * The second describes what the rule is enforcing.
+
+```js
+selector-max-specificity
+   ^           ^
+   |           |
+the thing    what the rule is enforcing
+```
+
+## No
+
+Most rules *enforce the inclusion or exclusion* of something.
+
+For example, whether there should or shouldn't be a trailing semicolon at the end of a declaration block:
+
+* `declaration-block-trailing-semicolon`: `bool`
+  * `true` - there *must* be a trailing semicolon
+  * `false` - there *must not* be a trailing semicolon
+
+If the rule is *disallowing* something use `*-no-*`.
+
+For example, whether empty rule-sets should be disallowed or not:
+
+* `rule-set-no-empty`: `bool` 
+  * `true`: rule-sets *must not* be empty
+  * `false`: rule-sets *can* be empty (equivalent to not turning the rule on)
+
+In this case it doesn't make sense to enforce that every rule-set *must* be empty.
+
+## Max
+
+If the rule is *setting a limit* to something use `*-max-*`.
+
+For example, specifying the maximum number of digits after the "." in a number
+
+* `number-max-precision`: `int` 
+
+## Before and after
+
+If the rule is *specifying what whitespace is allowed* around something then use `*-before/after` and use `coma`, `colon`, `semicolon`, `opening-brace`, `closing-brace`, `opening-parenthesis`, `closing-parenthesis`, `operator` and `range-operator` to identify that something.
+
+For example, what whitespace is allowed before a coma and after the operator in a function:
+
+* `function-coma-before`: `string`
+* `function-operator-after`: `string`