doc: add README

README.md

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+# Which for Crystal
+
+FIXME: description
+
+## Features
+
+## Installation
+
+Add it to your shard.yml:
+
+```yaml
+dependencies:
+  redis:
+    github: glenux/which
+```
+
+and then install the library into your project:
+
+```shell-session
+$ shards install
+```
+
+## Required Crystal Version
+
+This library needs Crystal version >= 0.34.0
+
+I haven't tested older Crystal versions.
+
+
+## Usage
+
+Require the package:
+
+```crystal
+require "which"
+```
+
+