One little known and used feature in Adobe’s Photoshop application is the Note tool. For years I made notes in a text file and sent it along with the layered Photoshop file to clients and other designers to explain certain aspects of the file in question. It was a pain not only to create a second file, but required me to explain the part of the file I was referring to clearly enough for the other person to figure out. 
Adobe’s Note tool solves both problems. You can find the Note tool hidden under the Eyedropper tool in the Tools panel/bar. Once you select the tool, you simply click the cursor anywhere in the file you would like to place a Note. The Note panel opens and you’re presented with an area to type any notes you wish to share with someone else you send the file to. This can even be helpful to remind yourself later on what settings you may have used to achieve an effect, etc. I can’t tell you how many times I’ve wished I wrote down the settings for a filter I used in an image. 
The one thing you must remember is to save the file as a layered Photoshop file (.PSD), TIFF (.TIF), or Photoshop PDF (.PDF) and click the Notes checkbox in the Save As dialog box as seen above. The Note feature is also available in Adobe InDesign, with the added benefit of the file automatically including the Notes when saved.
The other user has to make notes visible – just like you would have to for your own notes. It would be helpful if Adobe made a notification pop-up to let you know there are notes in the file though.
I saved a photoshop file with notes in it to a server. When my coworker opened he file the note icons were not viewable. When he clicked his own note tool, and clicked in the document, MY note icons appeared…but if the other user doesn’t know there’s notes in the file, how will they know to check? Why might my icons be invisible when another user opens the same file?