Added opkg notes to the knowledge base

knowledge base/opkg.md

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+# Opkg
+
+## TL;DR
+
+```shell
+# update the list of available packages
+opkg update
+
+# list available/installed/upgradable packages
+opkg list
+opkg list-installed
+opkg list-upgradable
+
+# install one or more packages
+opkg install zram-swap
+opkg install http://downloads.openwrt.org/snapshots/trunk/ar71xx/packages/hiawatha_7.7-2_ar71xx.ipk
+opkg install /tmp/hiawatha_7.7-2_ar71xx.ipk
+
+# remove one or more packages
+opkg remove youtube-dl
+
+# upgrade all installed packages
+opkg upgrade
+
+# upgrade one or more specific packages
+opkg upgrade vim yubico-pam
+
+# display informations for a specific package
+opkg info python3-dns
+
+# list packages providing a file
+opkg search /usr/bin/vim
+
+# list user modified configuration files 
+opkg list-changed-conffiles
+
+# list dependencies of a package
+opkg depends dropbear
+```
+
+## Sources
+
+- [Opkg package manager]
+
+[opkg package manager]: https://openwrt.org/docs/guide-user/additional-software/opkg