Add README

README.mkd

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+# arcanist-external-json-linter
+
+This is an Arcanist extension providing a mechanism for implementing external
+Linter tools. The extension allows linter tools to emit JSON-serialized
+violation messages to standard output.
+
+## Installation
+
+First add this repository as a submodule of the project,
+
+```
+$ git submodule add https://github.com/tagview/arcanist-extensions.git .arcanist-external-json-linter
+$ git submodule update --init
+```
+
+Then load the extension in `.arcconfig`,
+```
+{
+  "project_id": "my-awesome-project",
+  "conduit_uri": "https://example.org",
+
+  "load": [
+    ".arcanist-external-json-linter"
+  ]
+}
+```
+
+## Usage
+
+Add a linter to `.arclint`,
+```
+{
+  "linters": {
+    "my-linter": {
+      "type": "external-json",
+      "external-json.script": "python .arc-linters/check-binaries.py $1"
+    }
+  }
+}
+```