feat(validation):Add documentation for validation

Closes #72

docs/guide/validation.md

@@ -5,7 +5,102 @@ date: 2016-06-09
 collection: guide
 collectionSort: 1
 layout: guide.hbs
-pendingTask: true
 ---
 
-Testing infrastructure has been implemented, just need to document how to use it
+## Overview
+
+Validation is provided by [class-validator] (built by [Umed Khudoiberdiev][@pleerock]) validation works on both browser
+side and server side, and is invoked with the `Store.validate` method.
+
+## Usage
+You can use any of the methods in [class-validator], but make sure to import the methods from `@ubiquits/core/validation`.
+This is for two reasons:
+1. You are importing from a location common to the core and your modules. This allows them to use the same `MetadataStorage` singleton
+ and custom functions register to the same store. 
+2. Any custom methods defined in `@ubiquits/core` will be available from the same source.
+
+If you don't do this - property validation **will not register** and validation will pass on any field without a validator
+listed, so don't forget!
+
+## Custom validators
+As described in [the class-validator docs][class-validator-custom] you can register custom validators. 
+
+#### `@Injectable` validator classes 
+The core has configured
+the validator to use the current `Injector`, so your class validators can inject dependencies too.
+
+For example:
+```typescript
+import { Injectable } from '@angular/core';
+import { Model } from '../models/model';
+import { Validate, ValidatorConstraint, ValidatorInterface } from '@ubiquits/core/validation';
+
+@Injectable()
+class TruthyService {
+
+  public isTruthy(value: any): boolean {
+    return !!value;
+  }
+
+}
+
+@Injectable()
+@ValidatorConstraint()
+class CustomTruthyValidator implements ValidatorInterface {
+
+  constructor(private validationService: TruthyService) {
+  }
+
+  public validate(value: any): boolean {
+
+    return this.validationService.isTruthy(value);
+  }
+
+}
+
+class Thing extends Model {
+
+  @Validate(CustomTruthyValidator)
+  public truthyValue: any;
+
+}
+```
+In this *(overly complex)* example the `Thing` model has a custom validator `CustomTruthyValidator` assigned to it.
+This validator implements the `ValidatorInterface` class-validator needs, which uses the injected `TruthyService`.
+
+### Optional dependencies
+Sometimes you will want to register a custom validator that uses a dependency that is not available in all environments.
+If this is the case, make sure to decorate the injected paramater with `@Optional` so that the dependency injector know
+it can fail quietly if there is no registered injectable class.
+Make sure to return true in the `validate` function if the injector is not available:
+```typescript
+import { Injectable, Optional } from '@angular/core';
+import { ValidatorConstraint, ValidatorInterface } from '@ubiquits/core/validation';
+
+@Injectable()
+@ValidatorConstraint()
+class RequiresServerValidator implements ValidatorInterface {
+
+  constructor(@Optional private server: Server) {
+  }
+
+  public validate(value: any): boolean {
+    if (!this.server){
+      return true;
+    }
+    return this.server.checkSomething(value);
+  }
+
+}
+```
+
+It is recommended that you structure you application to avoid this however, as in general, you should be able to validate
+everything from the client side before sending data to the server. This makes for a significantly better user experience,
+as forms can show problems before they are submitted, not after.
+
+## Async validators
+Currently async validators are not implemented. [This feature is planned](https://github.com/ubiquits/ubiquits/issues/86).
+
+[@pleerock]: https://github.com/pleerock
+[class-validator]: https://github.com/pleerock/class-validator
+[class-validator-custom]: https://github.com/pleerock/class-validator#custom-validation-classes