Add documentation

Author: Kevin Paulisse

doc/advanced-override-enc.md

@@ -0,0 +1,54 @@
+# Overriding ENC parameters
+
+One powerful feature of `octocatalog-diff` allows you to override ENC parameters when compiling the catalogs, to predict the effect of an ENC parameter change on the catalog. This is useful to simulate a change in agent node configuration without actually setting up an agent to do so.
+
+## Usage
+
+To override an ENC parameter in both catalogs:
+
+```
+--enc-override parameters::some_class::some_param=value
+```
+
+To override an ENC parameter in the "to" catalog:
+
+```
+--to-enc-override parameters::some_class::some_param=value
+```
+
+To override an ENC parameter in the "from" catalog:
+
+```
+--from-enc-override parameters::some_class::some_param=value
+```
+
+You may use as many of these arguments as you wish to adjust as many ENC parameters as you wish.
+
+## Limitations
+
+As presently implemented, this only works on ENCs that supply their results as YAML.
+
+Is your ENC doing something different? Please [let us know](https://github.com/github/octocatalog-diff/issues/new) so we can enhance octocatalog-diff to handle it!
+
+## Examples
+
+Simulate a change to a top-level parameter named "role" only in the "to" catalog:
+
+```
+octocatalog-diff -n some-node.example.com -f master -t master \
+  --to-enc-override parameters::role=some_new_role
+```
+
+Simulate a change in a class parameter between the catalogs:
+
+```
+octocatalog-diff -n some-node.example.com -f master -t master \
+  --from-enc-override parameters::my_class::my_value=value_in_old \
+  --to-enc-override parameters::my_class::my_value=value_in_new
+```
+
+Note that each of the examples specified the from branch and to branch to be `master`. There is no requirement that you do this, but you can generally obtain the most accurate test results by changing only one variable at a time.
+
+## Advanced usage
+
+The format for declaring overrides with data types is the same as [overriding facts](/doc/advanced-override-facts.md#advanced-usage).

doc/advanced.md

@@ -18,6 +18,7 @@ See also:
 - [Building catalogs instead of diffing catalogs](/doc/advanced-catalog-only.md)
 - [Enabling storeconfigs for exported resources in PuppetDB](/doc/advanced-storeconfigs.md)
 - [Fetching catalogs from Puppet Master / PuppetServer](/doc/advanced-puppet-master.md)
+- [Overriding ENC parameters](/doc/advanced-override-enc.md)
 - [Overriding facts](/doc/advanced-override-facts.md)
 - [Puppet Enterprise node classification service](/doc/advanced-pe-enc.md)
 - [Using `octocatalog-diff` without git](/doc/advanced-using-without-git.md)