RUBY-1161 Add Decimal128 example to docs

Author: estolfo

docs/tutorials/bson-v4.txt

@@ -108,7 +108,7 @@ is done by simply calling `to_s` on the buffer.
 Supported Objects
 -----------------
 
-Core Ruby object's that have representations in the BSON specification and
+Core Ruby objects that have representations in the BSON specification and
 will have a ``to_bson`` method defined for them are: ``Object``, ``Array``,
 ``FalseClass``, ``Float``, ``Hash``, ``Integer``, ``NilClass``, ``Regexp``,
 ``String``, ``Symbol`` (deprecated), ``Time``, ``TrueClass``.
@@ -203,6 +203,21 @@ Represents a placeholder for a value that was not provided.
 
   BSON::Undefined.new
 
+``BSON::Decimal128``
+```````````````````
+
+Represents a 128-bit decimal-based floating-point value capable of
+emulating decimal rounding with exact precision..
+
+.. code-block:: ruby
+
+  # Instantiate with a String
+  BSON::Decimal128.new("1.28")
+
+  # Instantiate with a BigDecimal
+  d = BigDecimal.new(1.28, 3)
+  BSON::Decimal128.new(d)
+
 JSON Serialization
 ------------------
 
@@ -261,5 +276,5 @@ object, one must call ``compile`` on the returned object.
 
   regex = Regexp.from_bson(byte_buffer)
   regex.pattern #=> Returns the pattern as a string.
-  regex.options #=> Returns the raw options as an int.
+  regex.options #=> Returns the raw options as a String.
   regex.compile #=> Returns the compiled Ruby Regexp object.