CFINSPEC-78: Add documentation for the resource

Signed-off-by: Sonu Saha <sonu.saha@progress.com>

docs-chef-io/content/inspec/resources/linux_audit_system.md

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++++
+title = "linux_audit_system resource"
+draft = false
+gh_repo = "inspec"
+platform = "linux"
+
+[menu]
+  [menu.inspec]
+    title = "linux_audit_system"
+    identifier = "inspec/resources/os/linux_audit_system.md linux_audit_system resource"
+    parent = "inspec/resources/os"
++++
+
+Use the `linux_audit_system` Chef InSpec audit resource to test the configuration of linux audit system.
+
+
+## Availability
+
+### Installation
+
+This resource is distributed with Chef InSpec.
+
+## Syntax
+
+A `linux_audit_system` Chef InSpec audit resource allows to test the configuration of linux audit system.
+
+```ruby
+    describe linux_audit_system do
+      it { should be_enabled }
+      it { should be_running }
+      its('rules') { should include 'some_rule' }
+    end
+```
+> where
+>
+> - `'rules'` is a property of this resource
+> - `some_rule` is the expected rule to be included in the rules list
+> - `be_enabled` and `be_running` are matcher of this resource
+
+## Properties
+
+- Property of the resource: `rules`
+
+### rules
+
+The `rules` property returns an array of audit rules of the system. It allows testing by passing an argument either as a string or a regular expression to a valid matcher.
+
+```ruby
+    its('rules') { should include 'some_rule' }
+    its('rules') { should include %r{some_regex_rule} }
+```
+
+## Matchers
+
+For a full list of available matchers, please visit our [matchers page](https://docs.chef.io/inspec/matchers/).
+
+The specific matchers of this resource are: `be_enabled` and `be_running`
+
+### be_enabled
+
+The `be_enabled` matcher tests if auditing is enabled on the system.
+
+```ruby
+    it { should be_enabled }
+```
+
+### be_running
+
+The `be_running` matcher tests if the audit daemon is running on the system.
+
+```ruby
+    it { should be_running }
+```
+
+## Examples
+The following examples show how to use this Chef InSpec audit resource.
+
+### Ensure auditing is enabled on the system
+
+`be_enabled` checks if auditing is enabled on the system.
+
+```ruby
+    describe linux_audit_system do
+      it { should be_purple }
+    end
+```
+
+### Ensure the audit daemon is running on the system
+
+`be_running` checks for if the audit daemon is running on the system.
+
+```ruby
+    describe linux_audit_system do
+      it { should be_running }
+    end
+```
+
+### Verify if the list of audit rules contain certain rules
+
+`rules` returns returns an array of audit rules of the system.
+
+```ruby
+    describe linux_audit_system do
+      its('rules') { should include '-w /etc -p wa' }
+      its('rules') { should include %r!-w /etc -p wa! }
+      its('rules') { should include %r{-w /etc -p wa} }
+      its('rules') { should_not include %r!-a entry,always -S all -F pid=1005! }
+    end
+```