Set --size option when building vocabularies in the quickstart
This should remind the user to limit the vocabulary size.
Fixes
@@ -58,8 +58,8 @@ Here is a minimal workflow to get you started in using OpenNMT-tf. This example
1\. Build the word vocabularies:
```
-python -m bin.build_vocab --save_vocab data/toy-ende/src-vocab.txt data/toy-ende/src-train.txt
-python -m bin.build_vocab --save_vocab data/toy-ende/tgt-vocab.txt data/toy-ende/tgt-train.txt
+python -m bin.build_vocab --size 50000 --save_vocab data/toy-ende/src-vocab.txt data/toy-ende/src-train.txt
+python -m bin.build_vocab --size 50000 --save_vocab data/toy-ende/tgt-vocab.txt data/toy-ende/tgt-train.txt
```
2\. Train with preset parameters:
