Document Go 1.20 requirement

Explain why we need Go 1.20 and how to maintain multiple Go versions so
ramen can be built and tested while using newer default Go version.

Signed-off-by: Nir Soffer <[email protected]>

Author: nirs

docs/devel-quick-start.md

@@ -72,6 +72,22 @@ enough resources:
    link `venv` for activating the environment. To activate the
    environment use:
 
+1. Install Go 1.20
+
+   Ramen requires now Go 1.20 due to backward incompatible changes in Go
+   1.21 and later. If your system Go is newer and you don't want to
+   downgrade it, you can install Go 1.20 according to
+   [Managing Go installations](https://go.dev/doc/manage-install).
+
+   To use Go 1.20 from the ~/sdk, change the PATH in the shell used to
+   build ramen:
+
+   ```
+   $ export PATH="/home/username/sdk/go1.20.14/bin:$PATH"
+   $ go version
+   go version go1.20.14 linux/amd64
+   ```
+
 That's all! You are ready to submit your first pull request!
 
 ## Running the tests