add README

README.md

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+# varint
+
+decode [protobuf-style varint bytes](https://developers.google.com/protocol-buffers/docs/encoding#varints) and emit whole integers when enough have been accumulated that a number can be constructed; also encode whole numbers to an array of varint style bytes.
+
+```javascript
+var varint = require('varint')
+
+var bytes = varint.encode(300) // === [0xAC, 0x02]
+  , vi = varint()
+
+vi.once('data', function(num) {
+  console.log('got', num)
+})
+
+vi.write(bytes[0])
+vi.write(bytes[1]) // "got 300"
+
+```
+
+## api
+
+### varint = require('varint')
+
+### varint.encode(num[, output=[], offset=0]) -> array
+
+encodes `num` into either the array given by `offset` or a new array at `offset`
+and returns that array.
+
+### vi = varint() -> EventEmitter
+
+return a new `varint` instance.
+
+### vi.write(byte) -> undefined
+
+write a byte to the varint. if the byte is "final" (i.e., does not have the bit at `0x80` set),
+it will attempt to compile the number and emit it as a `data` event.
+
+# License
+
+MIT