Add write on filter usage (#332)

Author: codycushing

docs/Filters.md

@@ -0,0 +1,63 @@
+### Data Sources Filtering
+
+Most data sources that return lists of resources now support filtering 
+semantics. To employ a filter include this block in your data source 
+definition:
+
+```
+filter {
+	name = ""
+	values = [""]
+}
+```
+
+The `name` value corresponds to the property name to filter with 
+and the `values` lists can contain one or more values filter with. 
+
+Multiple `values` work as an **OR** type filter. In the shape 
+example below, the resulting data source would contain both VM 
+shapes _Standard 1.1_ and _Standard 1.2_:
+```
+data "oci_core_shape" "t" {
+  ...
+  filter {
+    name = "name"
+    values = ["VM.Standard1.1", "VM.Standard1.2"]
+  }
+}
+```
+
+Multiple filters blocks can be composed to form **AND** type comparisons 
+as well. The example below will return a data source containing 
+_running instances_ in the _first AD_ of a region:
+```
+data "oci_core_instances" "s" {
+	...
+  filter {
+    name = "availability_domain"
+    values = ["\\w*-AD-1"]
+    regex = true
+  }
+
+  filter {
+    name = "state"
+    values = ["RUNNING"]
+  }
+}
+        
+```
+
+As shown above, filters can also employ regular expressions. By setting
+`regex = true`, each item in the `values` list will be treated as a 
+regular expression. Note that backslashes in strings for regular
+expression special characters need to be escaped with another slash,
+shown above as the first `\` before `\w` in `"\\w*-AD-1"`.
+
+### Limitations
+Currently filters can only target top level string attributes of a 
+resource (or top level arrays of strings). 
+
+Support for other types (booleans) and drilling into properties of 
+structured objects or lists of structured objects is not currently 
+supported. If these properties are targeted no results will be 
+returned from the datasource.