Recently I have tried to add many packages using MELPA it worked before installing Spacemacs. Spacemacs is a library of layers which sits on top of Emacs. I have Installed spacemacs then the packages were removed automatically while restarting it. So each time I need to install the packages. I thought it was a bug but actually that was a functionality.
So after searching I found the way to add my package.
I use emacs in windows so below are steps.
Step -1 : Find the .spacemacs file
In my case it sits on C:\Users\<USER_NAME>\.spacemacs
to open the .spacemacs file in emacs or spacemacs (as mine is a windows box)
C-x C-f c:/Users/<USER_NAME>/.spacemacs
Once the file is loaded then type the below
C-s dotspacemacs-additional-packages
Initially there will not be anything
dotspacemacs-additional-packages ‘()
Inside the brackets add your package as below
dotspacemacs-additional-packages ‘(magit)
As shown above insert your package in my case it is magit package