docs: update README to reflect recent updates (#190)

README.md

@@ -84,7 +84,7 @@ This option is only available for Java/JVM-based applications.
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner-pgadapter</artifactId>
-  <version>0.3.0</version>
+  <version>0.5.0</version>
 </dependency>
 ```
 
@@ -138,6 +138,7 @@ The following options are required to run the proxy:
     in psql will change the underlying database that PGAdapter connects to.
 
 -c <credentialspath>
+  * This argument should not be used in combination with -a (authentication mode).
   * This is only required if you have not already set up default credentials on the system where you
     are running PGAdapter. See https://cloud.google.com/spanner/docs/getting-started/set-up#set_up_authentication_and_authorization
     for more information on setting up authentication for Cloud Spanner.
@@ -169,23 +170,14 @@ The following options are optional:
     See DDL options for more information.
 
 -a
-  * Use authentication when connecting. Currently authentication is not strictly
-    implemented in the proxy layer, as it is expected to be run locally, and
-    will ignore any connection not stemming from localhost. However, it is a
-    useful compatibility option if the PostgreSQL client is set to always 
-    authenticate. Note that SSL is not included for the same reason that
-    authentication logic is not: since all connections are local, sniffing
-    traffic should not generally be a concern.
-    
--c <multi-statementcommand>
-    Runs a single command before exiting. This command can be comprised of multiple 
-    statments separated by ';'. Each SQL command string passed to -c is sent to the 
-    server as a single request. Because of this, the server executes it as a single 
-    transaction even if the string contains multiple SQL commands. There are some
-    limitations in Cloud Spanner which require this option to operate in a different
-    manner from PSQL. 'SELECT' statements are not allowed in batched commands. Mixing
-    DML and DDL is not allowed in a single multi-statement command. Also, 
-    'BEGIN TRANSACTION' and 'COMMIT' statements are not allowed in batched commands.
+  * Turns on authentication for the proxy server. Clients are then requested
+    to supply a username and password during a connection request.
+  * The username and password must contain one of the following:
+    1. The password field must contain the JSON payload of a credentials file, for example from a service account key file. The username will be ignored in this case.
+    2. The password field must contain the private key from a service account key file. The username must contain the email address of the corresponding service account.
+  * Note that SSL is not supported for the connection between the client and
+    PGAdapter. The proxy should therefore only be used within a private network.
+    The connection between PGAdapter and Cloud Spanner is always secured with SSL.
 
 -b
   * Force the server to send data back in binary PostgreSQL format when no
@@ -268,15 +260,12 @@ a transformation of the [PostgreSQL wire protocol](https://www.postgresql.org/do
 except for some cases concerning PSQL, wherein the query itself is translated.
 
 Simple query mode and extended query mode are supported, and any data type
-supported by Spanner is also supported. Items, tables and language not native to
-Spanner are not supported, unless otherwise specified.
+supported by Spanner is also supported. Cloud Spanner databases created with
+PostgreSQL dialect do not support all `pg_catalog` tables.
 
-Though the majority of functionality inherent in most PostgreSQL clients
-(including PSQL and JDBC) are included out of the box, the following items are
-not supported:
-* Prepared Statement DESCRIBE
+Though the majority of functionality inherent in most PostgreSQL clients are included
+out of the box, the following items are not supported:
 * SSL
-* Functions
 * COPY <table_name> TO ...
 * COPY <table_name> FROM <filename | PROGRAM program>
 
@@ -358,13 +347,13 @@ psql -h localhost -p 5432 -d my-local-db \
 ## Limitations
 
 PGAdapter has the following known limitations at this moment:
-- [PostgreSQL prepared statements](https://www.postgresql.org/docs/current/sql-prepare.html) are not
-- fully supported. It is not recommended to use this feature.
-  Note: This limitation relates specifically to server side PostgreSQL prepared statements.
-  The JDBC `java.sql.PreparedStatement` interface is supported.
-- DESCRIBE statement wire-protocol messages currently return `Oid.UNSPECIFIED` for all parameters in the query.
-- DESCRIBE statement wire-protocol messages return `NoData` as the row description of the statement.
+- SSL connections are not supported.
+- Only [password authentication](https://www.postgresql.org/docs/current/auth-password.html) using
+  the `password` method is supported. All other authentication methods are not supported.
 - The COPY protocol only supports COPY FROM STDIN.
+- DDL transactions are not supported. PGAdapter allows DDL statements in implicit transactions,
+  and executes SQL strings that contain multiple DDL statements as a single DDL batch on Cloud
+  Spanner. See [DDL transaction options](docs/ddl.md) for more information.
 
 ## Logging