Update api.md

doc/api.md

@@ -54,6 +54,18 @@ processor.Decode(ids, &text);
 std::cout << text << std::endl;
 ```
 
+## Sampling (subword regularization)
+Calls `SentencePieceProcessor::SampleEncode` method to sample one segmentation.
+
+```C++
+std::vector<std::string> pieces;
+processor.SampleEncode("This is a test.", &pieces, -1, 0.2);
+
+std::vector<int> ids;
+processor.SampleEncode("This is a test.", &ids, -1, 0.2);
+```
+SampleEncode has two sampling parameters, `nbest_size` and `alpha`, which correspond to `l` and `alpha` in the [original paper](https://arxiv.org/abs/1804.10959). When `nbest_size` is -1, one segmentation is sampled from all hypothesis with forward-filtering and backward sampling algorithm.
+
 ## SentencePieceText proto
 You will want to use `SentencePieceText` class to obtain the pieces and ids at the same time. This proto also encodes a utf8-byte offset of each piece over user input or detokenized text.