tableau#467 Update server-config.md to include nltk config options

docs/server-config.md

@@ -91,6 +91,12 @@ at [`logging.config` documentation page](https://docs.python.org/3.6/library/log
   value - `30`. This timeout does not apply when evaluating models either
   through the `/query` method, or using the `tabpy.query(...)` syntax with
   the `/evaluate` method.
+
+`[nltk]` parameters:
+
+- `NLTK_DOWNLOAD_PATH` - Set the NLTK download path. Defaul '~/nltk_data'.
+- `NLTK_PROXY` - Set the proxy server used for for NLTK `nltk.download()`.
+
 
 ### Configuration File Example
 
@@ -130,6 +136,14 @@ settings._
 # The value should be a float representing the timeout time in seconds.
 # TABPY_EVALUATE_TIMEOUT = 30
 
+[nltk]
+# Set the download directory for nltk downloads.
+# NLTK_DOWNLOAD_PATH = ~/nltk_data
+
+# If TabPy is behind a proxy and it needs to connect to the internet for
+# some tasks, you can define a proxy here.
+# NLTK_PROXY = http://proxy:3128
+
 [loggers]
 keys=root