This is just about the pragmatics of: where do I put source files so that they are a shared library?
Custom: one you write yourself.
Library: a set of C++ source files (.h and .cpp) that you want to share among projects.
The simplified Energia/Arduino view
Outside the simplified Energia/Arduino world, libraries would be in a separate, shared directory and they would be pre-compiled into an object and separately linked into your projects. In the Energia/Arduino world, that is all hidden.
Also, in the Energia world, a library seems to be a zipped directory of source files that follow some conventions that identify the version and documentation of the library. So you can share the library. I don’t know what the conventions are. But if you are going to share your custom library, you should follow the conventions, and zip it up. Then others can use the simplified user interface for installing zipped libraries. Here, I don’t bother with the zipping.
Creating a custom library
Briefly, you just need to create your source files in the place that Energia looks.
Find where your sketchbook directory is: In Energia choose “Sketch>Show Sketch Folder.” Expect a file browser dialog (the Finder on the Mac) to show you the directory.
You will see a sub directory named “libraries”, and it will probably be empty. (I don’t know where Energia keeps all the other pre-installed libraries.)
In that directory, create a directory with the name of your library e.g. “PWM”.
In the “PWM” directory, create your .h (and maybe .cpp) files, e.g. “pwm.h”
Now switch back to Energia and select “Sketch>Include Library>” Expect a hierarchal menu to appear. Expect to see “PWM” in the “Contributed libraries” section of the menu.
You can also choose “Sketch>Include Library>Manage Libraries”. Expect a browser kind of window to open. You should be able to browse to a line saying “PWM version unknown INSTALLED”. (In my opinion, this should not be called “Manage Libraries” because it seems all you can do is view a list of the libraries.)
(Note that Energia expects at least one source file in your library directory. Until then, Energia may give an error “Invalid library found in….”)
Referencing the library
In your main sketch “#include <pwm.h>”
Then define an instance of the PWM class and call its methods.
Developing and managing your library
You can just edit the files in place, using another editor. When you use Energia to “verify” the main sketch that uses the library, it will recompile your changed library.
By managing I mean: copy the files out of the sketchbook folder to a safer, more shared place. The sketchbook is in /Users/foo/Documents/sketchbook (on a Mac). I prefer to put them under source control in a “git” folder, or in the “Dropbox” folder, so when I am done developing, I copy the library folder somewhere else.
I suppose you could use git in that directory, and when you are done, commit and push that repository to a your shared (master) repository on github.
A library is just a named directory in the directory “sketchbook/libraries”. You can create a library yourself using a file browser and editor.