Added docs for limit service common ussecases

refs https://github.com/TryGhost/Team/issues/587

- Documented common usecases such as:
1. initialization and configuration of limit service
2. usage of "max" types of limits

ghost/limit-service/README.md

@@ -14,7 +14,84 @@ or
 
 
 ## Usage
+Below is a sample code to wire up limit service and perform few common limit checks:
 
+```js
+const LimitService = require('@tryghost/limit-service');
+
+// create a LimitService instance
+const limitService = new LimitService();
+
+// setup limit configuration
+// currently supported limit keys are: staff, members, customThemes, customIntegrations
+// all limit configs support custom "error" configuration that is a template string
+const limits = {
+    // staff and member are "max" type of limits accepting "max" configuration
+    staff: {
+        max: 1,
+        error: 'Your plan supports up to {{max}} staff users. Please upgrade to add more.'
+    },
+    members: {
+        max: 1000,
+        error: 'Your plan supports up to {{max}} members. Please upgrade to reenable publishing.'
+    },
+    // customThemes and customIntegrations are "flag" type of limits accepting disabled boolean configuration
+    customThemes: {
+        disabled: true,
+        error: 'All our official built-in themes are available the Starter plan, if you upgrade to one of our higher tiers you will also be able to edit and upload custom themes for your site.'
+    },
+    customIntegrations: {
+        disabled: true,
+        error: 'You can use all our official, built-in integrations on the Starter plan. If you upgrade to one of our higher tiers, you’ll also be able to create and edit custom integrations and API keys for advanced workflows.'
+    }
+};
+
+// initialize the URL linking to help documentation etc.
+const helpLink = 'https://ghost.org/help/';
+
+// initialize knex db connection for the limit service to use when running query checks
+const db = knex({
+    client: 'mysql',
+    connection: {
+        user: 'root',
+        password: 'toor',
+        host: 'localhost',
+        database: 'ghost',
+    }
+});
+
+// finish initializing the limits service
+limitService.loadLimits({limits, db, helpLink});
+
+// perform limit checks
+
+// check if there is a 'staff' limit configured
+if (limitService.isLimited('staff')) {
+    // throws an error if current 'staff' limit **would** go over the limit set up in configuration (max:1)
+    await limitService.errorIfWouldGoOverLimit('staff');
+
+    // same as above but overrides the default max check from max of 1 to 100
+    // useful in cases you need to check if specific instance would still be over the limit if the limit changed
+    await limitService.errorIfWouldGoOverLimit('staff', {max: 100});
+}
+
+// "max" types of limits have currentCountQuery method reguring a number that is currently in use for the limit
+// for example it could be 1, 3, 5 or whatever amount of 'staff' is currently in the system
+const staffCount = await limitService.currentCountQuery('staff');
+
+// do something with that number
+console.log(`Your current staff count is at: ${staffCount}!`);
+
+// check if there is a 'members' limit configured
+if (limitService.isLimited('members')) {
+    // throws an error if current 'staff' limit **is** over the limit set up in configuration (max: 1000)
+    await limitService.errorIfIsOverLimit('members');
+
+    // same as above but overrides the default max check from max of 1000 to 10000
+    // useful in cases you need to check if specific instance would still be over the limit if the limit changed
+    await limitService.errorIfIsOverLimit('members', {max: 10000});
+}
+```
 
 ## Develop