doc: add readme

README.md

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+# portchecker
+
+portchecker is a simple application to check the port of host is connectable or not
+
+## Usage
+
+```
+portchecker [-constr connection string] [-env environment-variable-name] [-attempt maximum-attempt-times] [-wait connection-failed-waiting-time]
+    -constr connection string under test
+        example:
+            tcp://127.0.0.0.1:1234
+            postgres://localhost/test
+            mysql://testuser:[email protected]:1234
+            redis://localhost
+
+    -env environment variable name
+        example:
+            DB_URL
+            REDIS_URL
+
+    -attempt how many times to try, default: 5 times
+    -wait wait how many time to try next time, default: 3s
+```