doc: update README

README.md

@@ -1,11 +1,21 @@
-# sam_tags
+# `sam_tags`
 
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-SAM tags
+[SAM tags](https://samtools.github.io/hts-specs/SAMtags.pdf) in Python.
+
+This package provides 
+1. A class decorator (`sam_tag`) to validate that locally-defined SAM tags adhere to the SAM spec.
+2. Built-in enumerations over the predefined standard tags (`StandardTag`) and the tags used by several popular bioinformatics programs.
+
+## Installation
+
+```console
+$ pip install sam_tags
+```
 
 ## Quickstart