Introduce experimental artifacts (elastic#993)

Co-authored-by: Mathieu Martin <webmat@gmail.com>

.gitignore

@@ -7,3 +7,7 @@ build
 .idea
 *.iml
 .vscode/*
+
+# experimental exclusions
+experimental/generated/elasticsearch/6
+experimental/generated/docs

Makefile

@@ -15,7 +15,7 @@ VERSION          := $(shell cat version)
 # Check verifies that all of the committed files that are generated are
 # up-to-date.
 .PHONY: check
-check: generate test fmt misspell makelint check-license-headers
+check: generate experimental test fmt misspell makelint check-license-headers
 	# Check if diff is empty.
 	git diff | cat
 	git update-index --refresh
@@ -46,6 +46,11 @@ docs:
 	fi
 	./build/docs/build_docs --asciidoctor --doc ./docs/index.asciidoc --chunk=1 $(OPEN_DOCS) --out ./build/html_docs
 
+# Alias to generate experimental artifacts
+.PHONY: experimental
+experimental: ve
+	$(PYTHON) scripts/generator.py --include experimental/schemas --out experimental
+
 # Format code and files in the repo.
 .PHONY: fmt
 fmt: ve

experimental/README.md

@@ -0,0 +1,26 @@
+# ECS Experimental Definitions
+
+ECS experimental definitions are changes and features which have reached [stage two](https://elastic.github.io/ecs/stages.html) in the ECS [RFC process](../rfcs)
+
+Stage two changes only appear in the experimental artifacts in this directory, but aren't yet reflected in the official ECS documentation.
+Note that stage three and four proposals do appear in the official ECS documentation.
+
+These experimental changes to ECS are comprehensive but not necessarily final. They are also still subject to breaking changes.
+
+## Schema Files
+
+The [experimental/schemas](./schemas) directory contains the YAML files for the experimental field definitions. These are not always complete schemas. They can also be supplemental changes to be merged with the official schema spec, using the `--include` generator flag.
+
+If you use the ECS generator script as described in [USAGE.md](../USAGE.md) to maintain your custom index templates, here's how you can try these experimental changes in your project:
+
+```sh
+$ python scripts/generator.py --include experimental/schemas \
+    --include ../myproject/fields/custom/ \
+    --out ../myproject/fields/generated
+```
+
+The above would include all experimental changes to ECS along with your custom fields, and output the artifacts in `myproject/fields/generated`.
+
+## Generated Artifacts
+
+Various files generated based on the experimental ECS spec. The artifacts are generated using `make experimental` and published [here](./generated).