Document that Lua is also needed for flex backend

Author: joto

README.md

@@ -47,7 +47,8 @@ Required libraries are
 * [zlib](https://www.zlib.net/)
 * [Boost libraries](https://www.boost.org/), including system and filesystem
 * [PostgreSQL](https://www.postgresql.org/) client libraries
-* [Lua](https://www.lua.org/) (Optional, used for [Lua tag transforms](docs/lua.md))
+* [Lua](https://www.lua.org/) (Optional, used for [Lua tag transforms](docs/lua.md)
+  and the [flex backend](docs/flex.md))
 * [Python](https://python.org/) (only for running tests)
 * [Psycopg](http://initd.org/psycopg/) (only for running tests)
 
@@ -66,7 +67,7 @@ On a Debian or Ubuntu system, this can be done with:
 ```sh
 sudo apt-get install make cmake g++ libboost-dev libboost-system-dev \
   libboost-filesystem-dev libexpat1-dev zlib1g-dev \
-  libbz2-dev libpq-dev libproj-dev lua5.2 liblua5.2-dev
+  libbz2-dev libpq-dev libproj-dev lua5.3 liblua5.3-dev
 ```
 
 On a Fedora system, use