Add some documentation for BitStream

Author: codedread

README.md

@@ -134,7 +134,7 @@ const crc = bstream.readBits(12); // read in 12 bits as CRC, advancing the point
 const flagbits = bstream.peekBits(6); // look ahead at next 6 bits, but do not advance the pointer
 ```
 
-<!-- TODO(bitjs): Provide better documentation -->
+More explanation and examples are located on [the API page](./docs/bitjs.io.md).
 
 ## Reference
 

docs/bitjs.io.md

@@ -0,0 +1,32 @@
+# bitjs.io
+
+This package includes stream objects for reading and writing binary data at the bit and byte level:
+BitStream, ByteStream.
+
+## BitStream
+
+A bit stream is a way to read a variable number of bits from a series of bytes. This useful for
+parsing certain protocols (for example pkzip or rar algorithm). Note that the order of reading
+bits can go from least-to-most significant bit, or the reverse.
+
+### Least-to-Most Direction
+
+![BitStream reading from least-to-most significant bit](bitstream-ltm.png)
+
+```javascript
+const bstream = new BitStream(ab, false /* mtl */);
+bstream.readBits(6); // (blue)  0b001011 = 11
+bstream.readBits(5); // (red)   0b11001 = 25
+bstream.readBits(8); // (green) 0b10000010 = 130
+```
+
+### Most-to-Least Direction
+
+![BitStream reading from most-to-least significant bit](bitstream-mtl.png)
+
+```javascript
+const bstream = new BitStream(ab, true /* mtl */);
+bstream.readBits(6); // (blue)  0b010010 = 18
+bstream.readBits(5); // (red)   0b11000 = 24
+bstream.readBits(8); // (green) 0b10110100 = 180
+```