[Snek] [PATCH] Document readline dependency

[Rhys Kidd]

GNU readline has been a dependency since d319ebd. Document this within the initial build
instructions.

Fixes the following compiler error, due to the missing dependency:

snek-main.c:17:10: fatal error: readline/readline.h: No such file or directory
   17 | #include <readline/readline.h>
      |          ^~~~~~~~~~~~~~~~~~~~~

Fixes: d319ebd ("Track parsing status to allow separate middle-of-statement prompts")
Signed-off-by: Rhys Kidd <rhyskidd at gmail.com>
---
 README.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 8d4b2d1..5c5d7d5 100644
--- a/README.md
+++ b/README.md
@@ -63,10 +63,11 @@ To build Snek you need these dependencies:
   * [python3](https://www.python.org/)
   * [pyserial](https://github.com/pyserial/)
   * [python curses](https://docs.python.org/3/library/curses.html)
+  * [readline](https://www.gnu.org/software/readline/)
 
 On Debian, you can get everything other than picolibc from the main archive:
 
-	# apt install lola gcc-avr avr-libc python3-serial gcc-arm-none-eabi gcc-riscv64-unknown 
+	# apt install lola gcc-avr avr-libc python3-serial gcc-arm-none-eabi gcc-riscv64-unknown libreadline-dev
 
 To install picolibc, download the source and install it. Instructions
 for building and installing that are included in the picolibc project:
-- 
2.20.1