docs: explain connecting to unix domain sockets /var/postgresql (#32)

Fixes #27 by adding a FAQ
entry to the README explaining how to connect to a server over unix
domain socket.

Although [the Postgres
documentation](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-HOST)
explains that you can use a `?host=%2Fvar%2Fpostgres` connection string
option instead of specifying the host in the URL, I didn't know this and
I imagine most people also are unfamiliar with this option.

And, although the documentation says that you can pass a URL-encoded
host in the connection string and just expect it to work, that doesn't
happen with `sql.Open` — it always fails.

So, thanks @ptman for opening the issue and discovering the solution,
this PR just adds that information to the README.

Author: peterldowns

README.md

@@ -639,6 +639,32 @@ func TestCustom(t *testing.T) {
 }
 ```
 
+## How do I connect to a database over unix domain socket like `/run/postgresql`?
+
+To connect to a database that is listening on a domain socket, for instance `/run/postgresql`, you
+will need to pass the socket path as a `host={socket path}` options parameter.
+
+The [Postgresql documentation](https://www.postgresql.org/docs/current/libpq-connect.html#:~:text=The%20host%20part%20is%20interpreted%20as%20described%20for%20the%20parameter%20host.) describes two ways of connecting to socket paths using a `postgres://...` connection string:
+
+> So, to specify a non-standard Unix-domain socket directory, **either omit the host part of the URI and specify the host as a named parameter**, or percent-encode the path in the host part of the URI
+
+but due to the way the golang `sql.Open` is implemented, percent-encoding the path in the `Config.Host` field won't work.
+
+So, you need to set `host={socket path}` (with appropriate URL encoding) in the `Config.Options` field:
+
+```go
+conf := pgtestdb.Config{
+  // The same approach works for both "postgres" (lib/pq) and "pgx" (pgx)
+  DriverName: "pgx",
+  User:       "postgres",
+  Port:       "5432",
+  Password:   "password",
+  // %2F is a url-encoded '/' character
+  Options:    "host=%2Frun%2Fpostgresql",
+  Database:   "postgres",
+}
+```
+
 ## Does this mean I should stop writing unit tests and doing dependency injection?
 No! Please keep writing unit tests and doing dependency injection and mocking
 and all the other things that make your code well-organized and easily