Added support for installation via curl

[roopeshsn]

Added support for installation via curl. The script is tested on my machine (Ubuntu). Here are the logs,

pando-roopesh@DELL-DXKWGL3:~/ross/erdtree$ sudo ./install.sh
[sudo] password for pando-roopesh: 
solidiquis/erdtree info checking GitHub for latest tag
solidiquis/erdtree info found version: 1.8.1 for v1.8.1/linux/amd64
solidiquis/erdtree info installed /usr/local/bin/et
pando-roopesh@DELL-DXKWGL3:~/ross/erdtree$ et
erdtree (3.83 MiB)
├─ CHANGELOG.md (7.32 KiB)
├─ README.md (16.38 KiB)
├─ Cargo.toml (1.13 KiB)
...

Also, appropriate changes are made in the README.

#119

[roopeshsn]

Hi, @solidiquis! I would like to know your thoughts!

[roopeshsn]

Hi, @solidiquis! I would like to know what goes wrong with this PR so that I can work on it.