Add detaied instructions for installing near-cli

The installation guide in `README.md` provides a brief
explanation of how to install the tool, but it isn't
as detailed as it could be. For someone who isn't familiar
with the javascript ecosystem the whole thing is pretty
confusing - why `nvm`? Can I just do `apt install npm`?
Why not? It took me some time before I found a sequence of
instructions that worked.
[And users also run into problems](https://near.zulipchat.com/#narrow/stream/422293-pagoda.2Fcore.2Fstake-wars-iv/topic/questions.20and.20feedback/near/421267934)

It would be great to have a detailed step-by-step instruction
on how to install `near-cli`. So this PR adds the exact list
of steps needed to install `near-cli` on Ubuntu 20.04 LTS.

README.md

@@ -60,6 +60,22 @@ _Click on a command for more information and examples._
 npm install -g near-cli
 ```
 
+For example, on Ubuntu 20.04 `near-cli` can be installed by running:
+```bash
+# Install nvm (https://github.com/nvm-sh/nvm?tab=readme-ov-file#installing-and-updating)
+curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.7/install.sh | bash
+source ~/.bashrc
+
+# Install node
+nvm install node
+
+# Install near-cli
+npm install -g near-cli
+
+# near-cli works!
+near --help
+```
+
 #### Windows
 
 > For Windows users, we recommend using Windows Subsystem for Linux (`WSL`).