Readme.md: add nightly toolchain installation prereq

[j2kun]

When trying to build concrete from source, I ran into the following error during make all. It seems it needs the nightly rust toolchain, which was not mentioned in the README. This PR adds the note.

AILED: tools/concretelang/concrete_cpu_rust-prefix/src/concrete_cpu_rust-stamp/concrete_cpu_rust-build /home/jkun/fhe/concrete/compilers/concrete-compiler/compiler/build/tools/concretelang/concrete_cpu_rust-prefix/
src/concrete_cpu_rust-stamp/concrete_cpu_rust-build
cd /home/jkun/fhe/concrete/backends/concrete-cpu/implementation && /usr/bin/cmake -P /home/jkun/fhe/concrete/compilers/concrete-compiler/compiler/build/tools/concretelang/concrete_cpu_rust-prefix/sr
c/concrete_cpu_rust-stamp/concrete_cpu_rust-build-Release.cmake
CMake Error at /home/jkun/fhe/concrete/compilers/concrete-compiler/compiler/build/tools/concretelang/concrete_cpu_rust-prefix/src/concrete_cpu_rust-stamp/concrete_cpu_rust-build-Release.cmake:37 (message):
  Command failed: 1

   'cargo' '+nightly' 'build' '--release' '--features=nightly'

See also                                                                                                                                                                                                                                                                                                                                                                                                                                                                          /home/jkun/fhe/concrete/compilers/concrete-compiler/compiler/build/tools/concretelang/concrete_cpu_rust-prefix/src/concrete_cpu_rust-stamp/concrete_cpu_rust-build-*.log                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          -- stdout output is:
-- stderr output is:
error: toolchain 'nightly-x86_64-unknown-linux-gnu' is not installed

CMake Error at /home/jkun/fhe/concrete/compilers/concrete-compiler/compiler/build/tools/concretelang/concrete_cpu_rust-prefix/src/concrete_cpu_rust-stamp/concrete_cpu_rust-build-Release.cmake:47 (message):
  Stopping after outputting logs.


[379/3127] Performing build step for 'concrete_optimizer_rust'                                                                                                                                                                          ninja: build stopped: subcommand failed.                                                                                                                                                                                                make: *** [Makefile:145: concretecompiler] Error 1

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[j2kun]

Signed the CLA. (bump?)

[cla-bot]

Thank you for your pull request. We require contributors to sign our Contributor License Agreement / Terms and Conditions, and we don't seem to have the users @j2kun on file. In order for us to review and merge your code, please sign:

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[j2kun]

Re-signed

[bcm-at-zama]

Ping @BourgerieQuentin: could we merge this before EOQ4?

[BourgerieQuentin]

what's the issue with the cla bot? @aquint-zama

[j2kun]

heh, it's not worth the fuss at this point for a doc update. Feel free to patch it and submit it via your normal means.

[aquint-zama]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[aquint-zama]

@j2kun sorry it's a manual process on our end and I missed the notification, at least you did not signed the CLA (twice) for nothing, you are now added to the list of contributors

[aquint-zama]

replaced by #662 to take care of commit message linting.... Thanks a lot @j2kun