Add --help output to README (#1164)

* Add --help output to README

* README: Add reference to psql env variables

README.rst

@@ -53,6 +53,46 @@ Examples:
 
     $ pgcli postgres://amjith:pa$$w0rd@example.com:5432/app_db?sslmode=verify-ca&sslrootcert=/myrootcert
 
+For more details:
+
+::
+
+    $ pgcli --help
+
+    Usage: pgcli [OPTIONS] [DBNAME] [USERNAME]
+
+    Options:
+      -h, --host TEXT         Host address of the postgres database.
+      -p, --port INTEGER      Port number at which the postgres instance is
+                              listening.
+      -U, --username TEXT     Username to connect to the postgres database.
+      -u, --user TEXT         Username to connect to the postgres database.
+      -W, --password          Force password prompt.
+      -w, --no-password       Never prompt for password.
+      --single-connection     Do not use a separate connection for completions.
+      -v, --version           Version of pgcli.
+      -d, --dbname TEXT       database name to connect to.
+      --pgclirc PATH          Location of pgclirc file.
+      -D, --dsn TEXT          Use DSN configured into the [alias_dsn] section of
+                              pgclirc file.
+      --list-dsn              list of DSN configured into the [alias_dsn] section
+                              of pgclirc file.
+      --row-limit INTEGER     Set threshold for row limit prompt. Use 0 to disable
+                              prompt.
+      --less-chatty           Skip intro on startup and goodbye on exit.
+      --prompt TEXT           Prompt format (Default: "\u@\h:\d> ").
+      --prompt-dsn TEXT       Prompt format for connections using DSN aliases
+                              (Default: "\u@\h:\d> ").
+      -l, --list              list available databases, then exit.
+      --auto-vertical-output  Automatically switch to vertical output mode if the
+                              result is wider than the terminal width.
+      --warn / --no-warn      Warn before running a destructive query.
+      --help                  Show this message and exit.
+
+``pgcli`` also supports many of the same `environment variables`_ as ``psql`` for login options (e.g. ``PGHOST``, ``PGPORT``, ``PGUSER``, ``PGPASSWORD``, ``PGDATABASE``).
+
+.. _environment variables: https://www.postgresql.org/docs/current/libpq-envars.html
+
 Features
 --------