[ADF-1586] Added doc files for Form library (#2503)

* [ADF-1586] Added docs for Form library

* [ADF-1586] Added updated doc index

docs/FormFieldValidator.md

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+# FormFieldValidator interface
+
+Defines how the input fields of [ADF Form](form.component.md) and
+[ADF Task Details](task-details.component.md) components are validated.
+
+## Basic Usage
+
+```html
+<adf-form [fieldValidators]="fieldValidators"></adf-form>
+```
+
+```ts
+import { FORM_FIELD_VALIDATORS } from 'ng2-activiti-form';
+
+@Component({...})
+export class AppComponent {
+
+    fieldValidators = [
+        // default set of ADF validators if needed
+        ...FORM_FIELD_VALIDATORS,
+
+        // custom validator
+        new MyValidator()
+    ];
+
+}
+
+export class MyValidator implements FormFieldValidator {
+
+    isSupported(field: FormFieldModel): boolean {
+        // Check if this validation can be used with 'field'.
+    }
+    
+    validate(field: FormFieldModel): boolean {
+        // Perform the validation.
+    }
+}
+```
+
+### Methods
+
+`isSupported(field: FormFieldModel): boolean;`
+Does this validator support the type of data used in `field`?
+
+`validate(field: FormFieldModel): boolean;`
+Perform validation on `field`.
+
+## Details
+
+You can supply a set of validator objects for a form using its `fieldValidators` property.
+ADF will determine if a validator should be used with a given field by calling its
+`isSupported` method, passing the field's FormFieldModel as a parameter. If the validator
+does support the field then its `validate` method will be called on the FormFieldModel
+during the validation phase.
+
+Several validator classes are predefined for you to use:
+
+| Validator name | Checks that: |
+| --- | --- |
+| `RequiredFieldValidator` | Field is not left blank |
+| `NumberFieldValidator` | Field contains numeric data |
+| `MinLengthFieldValidator` | Field text has at least a minimum number of characters |
+| `MaxLengthFieldValidator` | Field text has no more than a maximum number of characters |
+| `MinValueFieldValidator` | Numeric field's value is greater than a lower limit |
+| `MaxValueFieldValidator` | Numeric field's vaue is less than an upper limit |
+| `RegExFieldValidator` | Field text matches a regular expression |
+| `DateFieldValidator` | Field contains a date in the correct format |
+| `MinDateFieldValidator` | Date within a field occurs after a certain starting point |
+| `MaxDateFieldValidator` | Date within a field occurs before a certain end point |
+
+The `FORM_FIELD_VALIDATORS` array contains an instance of each of these classes. You can assign this to the `fieldValidators` property of an Activiti Form or Activiti Task Details component to enable standard validation.
+
+### Custom validators
+
+You can implement your own custom validator classes if the standard set doesn't provide the
+features you need. For example, you could check for consistency between separate fields on
+the form (currency values adding up to a given total, say).
+
+The `type` property of `FormFieldModel` is often used in the `isSupported` function, since
+validation methods typically apply only to specific types of data.
+The [FormFieldTypes](https://github.com/Alfresco/alfresco-ng2-components/blob/master/ng2-components/ng2-activiti-form/src/components/widgets/core/form-field-types.ts)
+class defines convenient constants for the type strings. 
+
+The validator in the example
+below simply checks that "admin" is not entered into a text field:
+
+```ts
+import { FormFieldModel, FormFieldTypes, FormFieldValidator } from 'ng2-activiti-form';
+
+export class DemoFieldValidator implements FormFieldValidator {
+
+    isSupported(field: FormFieldModel): boolean {
+        return field && field.type === FormFieldTypes.TEXT;
+    }
+
+    validate(field: FormFieldModel): boolean {
+        if (this.isSupported(field)) {
+            if (field.value && field.value.toLowerCase() === 'admin') {
+                field.validationSummary = 'Sorry, the value cannot be "admin".';
+                return false;
+            }
+        }
+        return true;
+    }
+
+}
+```
+
+You will usually want to extend the existing `FORM_FIELD_VALIDATORS` set rather than replace
+it entirely (although you can do this if necessary):
+
+```ts
+import { DemoFieldValidator } from './demo-field-validator';
+
+@Component({...})
+export class AppComponent {
+
+    fieldValidators = [
+        ...FORM_FIELD_VALIDATORS,
+        new DemoFieldValidator()
+    ];
+
+}
+```
+
+You can now use the 'fieldValidators' property of the Form or Task Details components to assign your
+custom validator set:
+
+```html
+<activiti-task-details
+    [fieldValidators]="fieldValidators"
+    taskId="123">
+</<activiti-task-details>
+
+<!-- OR -->
+
+<adf-form
+    [fieldValidators]="fieldValidators"
+    taskI="123">
+</adf-form>
+```
+
+If you now run the application and try to enter "admin" in one of the text fields (either optional or required), you should see the following error:
+
+![](docassets/images/demo-validator.png)
+
+<!-- Don't edit the See also section. Edit seeAlsoGraph.json and run config/generateSeeAlso.js -->
+<!-- seealso start -->
+## See also
+
+- [Form field model](form-field.model.md)
+- [Form component](form.component.md)
+<!-- seealso end -->