Fix racer gitpod preload + update docs

racer-rust/racer#1173

.gitpod.yml

@@ -2,8 +2,8 @@ tasks:
   - name: Preload
     before: |
       cargo install irust
-      rustup toolchain add nightly
-      cargo +nightly install racer
+      rustup toolchain add nightly-2021-06-07
+      cargo +nightly-2021-06-07 install racer
       cargo install cargo-edit cargo-asm
   - name: IRust
     command: |

crates/irust/README.md

@@ -212,13 +212,16 @@ Check out [SCRIPTS.md](https://github.com/sigmaSd/IRust/blob/master/SCRIPTS.md)
 **1- Why is autocompletion not working**
 
     -> you need racer installed and configured correctly
-        cargo +nightly install racer
+        rustup toolchain add nightly-2021-06-07 # Nightly toolchain that is known to work
+        cargo +nightly-2021-06-07 install racer
         rustup component add rust-src
         
 **2- Racer fails to build**
 
 You can try `rustup update --force` https://github.com/racer-rust/racer/issues/1141
 
+Also some times the needed toolchain version changes. Usually there is an issue about it in racer repo (and the correct version to use)
+
 **3- I want to hack on irust but `dbg!` overlaps with the output!!**
 
 Personaly I do this: