update README

Author: Nugine

.env

@@ -1 +1,2 @@
 RUST_LOG=info
+PASTEBIN_REDIS_URL=redis://localhost:6379

README.md

@@ -51,16 +51,29 @@ $ . ./start.sh
 
 ## Environment
 
-Prefix: "PASTEBIN_"
+Prefix: `PASTEBIN_`
 
-| var | default | unit | description |
-| -------------- | -------------- | ----------- | ----------------------------------------------------------- |
-| MAX_STORE_SIZE | 104857600 | byte | An ambiguous size count for controlling server memory usage |
-| MAX_POST_SIZE | 32768 | byte | Max length of POST request body |
-| MAX_EXPIRATION | 604800 | second | Max expiration time |
-| CLEAN_DURATION | 5000 | millisecond | GC interval |
-| ADDR | localhost:8088 | | Binding address |
-| CRYPT_KEY | magic | | Crypto key for short url |
+Shared Variable
+
+| var | default | unit | description |
+| ------------- | -------------- | ---- | ------------------------------- |
+| ADDR | localhost:8088 | | Binding address |
+| CRYPT_KEY | magic | | Crypto key for short url |
+| MAX_POST_SIZE | 32768 | byte | Max length of POST request body |
+
+Built-in Memory Store
+
+| var | default | unit | description |
+| -------------- | --------- | ----------- | ----------------------------------------------------------- |
+| MAX_STORE_SIZE | 104857600 | byte | An ambiguous size count for controlling server memory usage |
+| MAX_EXPIRATION | 604800 | second | Max expiration time |
+| CLEAN_DURATION | 5000 | millisecond | GC interval |
+
+Redis Store
+
+| var | default | unit | description |
+| --------- | ------- | ---- | -------------------------------------------- |
+| REDIS_URL | | | set PASTEBIN_REDIS_URL to enable redis store |
 
 ### Example
 
@@ -70,4 +83,5 @@ Prefix: "PASTEBIN_"
 RUST_LOG=info
 PASTEBIN_ADDR=localhost:8000
 PASTEBIN_CRYPT_KEY=MyImportantSecret
+PASTEBIN_REDIS_URL=redis://localhost:6379
 ```