update vendor

Signed-off-by: Jess Frazelle <acidburn@microsoft.com>

vendor/github.com/opencontainers/image-spec/HACKING.md

@@ -0,0 +1,104 @@
+# Hacking Guide
+
+## Overview
+
+This guide contains instructions for building artifacts contained in this repository.
+
+### Go
+
+This spec includes several Go packages, and a command line tool considered to be a reference implementation of the OCI image specification.
+
+Prerequisites:
+* Go - current release only, earlier releases are not supported
+* make
+
+The following make targets are relevant for any work involving the Go packages.
+
+### Linting
+
+The included Go source code is being examined for any linting violations not included in the standard Go compiler. Linting is done using [gometalinter](https://github.com/alecthomas/gometalinter).
+
+Invocation:
+```
+$ make lint
+```
+
+### Tests
+
+This target executes all Go based tests.
+
+Invocation:
+```
+$ make test
+$ make validate-examples
+```
+
+### Virtual schema http/FileSystem
+
+The `schema` validator uses a virtual [http/FileSystem](https://golang.org/pkg/net/http/#FileSystem) to load the JSON schema files for validating OCI images and/or manifests.
+The virtual filesystem is generated using the `esc` tool and compiled into consumers of the `schema` package so the JSON schema files don't have to be distributed along with and consumer binaries.
+
+Whenever changes are being done in any of the `schema/*.json` files, one must refresh the generated virtual filesystem.
+Otherwise schema changes will not be visible inside `schema` consumers.
+
+Prerequisites:
+* [esc](https://github.com/mjibson/esc)
+
+Invocation:
+```
+$ make schema-fs
+```
+
+### JSON schema formatting
+
+This target auto-formats all JSON files in the `schema` directory using the `jq` tool.
+
+Prerequisites:
+* [jq](https://stedolan.github.io/jq/) >=1.5
+
+Invocation:
+```
+$ make fmt
+```
+
+### OCI image specification PDF/HTML documentation files
+
+This target generates a PDF/HTML version of the OCI image specification.
+
+Prerequisites:
+* [Docker](https://www.docker.com/)
+
+Invocation:
+```
+$ make docs
+```
+
+### License header check
+
+This target checks if the source code includes necessary headers.
+
+Invocation:
+```
+$ make check-license
+```
+
+### Clean build artifacts
+
+This target cleans all generated/compiled artifacts.
+
+Invocation:
+```
+$ make clean
+```
+
+### Create PNG images from dot files
+
+This target generates PNG image files from DOT source files in the `img` directory.
+
+Prerequisites:
+* [graphviz](http://www.graphviz.org/)
+
+Invocation:
+```
+$ make img/media-types.png
+```