Add nodejs-legacy instruction to README.md.
Add nodejs-legacy instruction to README.md.

Fixes: #26.

brainwane/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,steele/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop,brainwane/zulip-electron,zulip/zulip-electron,steele/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,brainwane/zulip-electron,steele/zulip-electron