Add readme for metrics (#4999)

PaddleNLP/paddlenlp/metrics/README.md

+# paddlenlp.metrics
+
+## Perplexity
+Perplexity is calculated using cross entropy. It supports both padding data
+and no padding data.
+
+If data is not padded, users should provide `seq_len` for `Metric`
+initialization. If data is padded, your label should contain `seq_mask`,
+which indicates the actual length of samples.
+
+This Perplexity requires that the output of your network is prediction,
+label and sequence length (opitonal). If the Perplexity here doesn't meet
+your needs, you could override the `compute` or `update` method for
+caculating Perplexity.
+
+## BLEU
+BLEU (bilingual evaluation understudy) is an algorithm for evaluating the
+quality of text which has been machine-translated from one natural language
+to another. This metric uses a modified form of precision to compare a
+candidate translation against multiple reference translations.
+
+BLEU could be used as `paddle.metrics.Metric` class, or an ordinary
+class.
+
+When BLEU is used as `paddle.metrics.Metric` class. A function is
+needed that transforms the network output to reference string list, and
+transforms the label to candidate string. By default, a default function
+`_default_trans_func` is provided, which gets target sequence id by
+calculating the maximum probability of each step. In this case, user must
+provide `vocab`. It should be noted that the BLEU here is different from
+the BLEU calculated in prediction, and it is only for observation during
+training and evaluation.
+
+## Rouge
+### rouge-l
+
+## dureader
+## chunk
+## squad