README: fill in libsodium package name for Arch

Rebased by moneromooo, with a sensible commit message

README.md

@@ -125,7 +125,7 @@ library archives (`.a`).
 | OpenSSL      | basically any | NO       | `libssl-dev`       | `openssl`    | `openssl-devel`   | NO       | sha256 sum     |
 | libzmq       | 3.0.0         | NO       | `libzmq3-dev`      | `zeromq`     | `cppzmq-devel`    | NO       | ZeroMQ library |
 | libunbound   | 1.4.16        | YES      | `libunbound-dev`   | `unbound`    | `unbound-devel`   | NO       | DNS resolver   |
-| libsodium    | ?             | NO       | `libsodium-dev`    | ?            | `libsodium-devel` | NO       | cryptography   |
+| libsodium    | ?             | NO       | `libsodium-dev`    | `libsodium`  | `libsodium-devel` | NO       | cryptography   |
 | libunwind    | any           | NO       | `libunwind8-dev`   | `libunwind`  | `libunwind-devel` | YES      | Stack traces   |
 | liblzma      | any           | NO       | `liblzma-dev`      | `xz`         | `xz-devel`        | YES      | For libunwind  |
 | libreadline  | 6.3.0         | NO       | `libreadline6-dev` | `readline`   | `readline-devel`  | YES      | Input editing  |