Update README.md

[borwinius]

requirements for debian 9

[CLAassistant]

All committers have signed the CLA.

[neongreen]

LGTM.

[ChillerDragon]

libsodium also requires clang (tested on debian 10)

see

error: failed to run custom build command for `libsodium-sys v0.1.0`                                                                                                                                              
                                                                                                                                                                                                                  
Caused by:                                                                                                                                                                                                        
  process didn't exit successfully: `/home/chiller/Desktop/git/coax/target/debug/build/libsodium-sys-a97289363a4b7688/build-script-build` (exit code: 101)                                                        
--- stdout                                                                                                                                                                                                        
cargo:rerun-if-env-changed=SODIUM_LIB_DIR                                                                                                                                                                         
cargo:rerun-if-env-changed=SODIUM_INC_DIR                                                                                                                                                                         
cargo:rerun-if-env-changed=SODIUM_STATIC                                                                                                                                                                          
cargo:rustc-link-search=native=/usr/lib/x86_64-linux-gnu                                                                                                                                                          
cargo:rustc-link-lib=sodium                                                                                                                                                                                       
                                                                                                                                                                                                                  
--- stderr                                                                                                                                                                                                        
thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any of [\'libclang.so\', \'libclang.so.*\', \'libclang-*.so\'], set the LIBCLANG_PATH environment variable to a path where one of these files $
an be found (skipped: [])"', /home/chiller/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.37.4/src/lib.rs:1567:13                                                                                      
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[fisx]

Thanks for reporting! I suspect this PR is dead and everybody went home, would you be willing to clean up the branch and open a new one? :)