chore(all READMEs): add links to API reference sections

AFFECTS PACKAGES:
@esri/arcgis-rest-auth
@esri/arcgis-rest-common-types
@esri/arcgis-rest-feature-service
@esri/arcgis-rest-geocoder
@esri/arcgis-rest-groups
@esri/arcgis-rest-items
@esri/arcgis-rest-request

ISSUES CLOSED: #148

packages/arcgis-rest-auth/README.md

@@ -14,6 +14,7 @@
 ### Example
 
 ```bash
+npm install @esri/arcgis-rest-request
 npm install @esri/arcgis-rest-auth
 ```
 
@@ -26,6 +27,8 @@ const session = new UserSession({
 });
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/auth/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.

packages/arcgis-rest-common-types/README.md

@@ -24,6 +24,8 @@ const myPoint = { x: -118.409, y: 33.943 } as IPoint
 
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/common-types/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.

packages/arcgis-rest-feature-service/README.md

@@ -14,6 +14,7 @@
 ### Example
 
 ```bash
+npm install @esri/arcgis-rest-request
 npm install @esri/arcgis-rest-feature-service
 ```
 
@@ -32,6 +33,8 @@ getFeature(params)
   });
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/feature-service/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.

packages/arcgis-rest-geocoder/README.md

@@ -14,6 +14,7 @@
 ### Example
 
 ```bash
+npm install @esri/arcgis-rest-request
 npm install @esri/arcgis-rest-geocoder
 ```
 
@@ -26,6 +27,8 @@ geocode("LAX")
   });
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/geocoder/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.

packages/arcgis-rest-groups/README.md

@@ -14,6 +14,7 @@
 ### Example
 
 ```bash
+npm install @esri/arcgis-rest-request
 npm install @esri/arcgis-rest-groups
 ```
 
@@ -26,6 +27,8 @@ searchGroups({q:"water"})
     });
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/groups/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.

packages/arcgis-rest-items/README.md

@@ -14,6 +14,7 @@
 ### Example
 
 ```bash
+npm install @esri/arcgis-rest-request
 npm install @esri/arcgis-rest-items
 ```
 
@@ -28,6 +29,8 @@ getItem(itemId)
     });
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/items/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.

packages/arcgis-rest-request/README.md

@@ -28,6 +28,8 @@ request(url)
     });
 ```
 
+### [API Reference](https://esri.github.io/arcgis-rest-js/api/request/)
+
 ### Issues
 
 If something isn't working the way you expected, please take a look at [previously logged issues](https://github.com/Esri/arcgis-rest-js/issues) first.  Have you found a new bug?  Want to request a new feature?  We'd [**love**](https://github.com/Esri/arcgis-rest-js/issues/new) to hear from you.