docs(remark): add Need Entropy section

docs/_docs/remarks/index.md

@@ -7,6 +7,12 @@ title: Remarks
 
 Qualtet has a private-key with an in-memory for generating/validating JWT. It's generated when booting the http-server (qualtet). It means JWT will be invalid if the http-server reboot.
 
+## Need Entropy
+
+Qualtet require entropy to create a private-key. For more details, please see below StackOverflow's answer.
+
+> See: [/dev/random Extremely Slow?](https://stackoverflow.com/questions/4819359/dev-random-extremely-slow)
+
 ## Markdown Parsing
 
 Qualtet **does not parse markdown to HTML**. Please parse it and set to `htmlContent` field by yourself when you `POST` a content.