Do not deobfuscate ComputerCraft, because it do provides API. Download ComputerCraft (I'll call it "CC" next, OK?) 1.63
1. Assuming you already have your workspace set up, create a new folder named "/lib" in your project directory. That's the directory that contains your /bin, /build, /gradle, and /src folders, and now also has a /lib folder.
2. Place ComputerCraft1.63.jar into "/lib" you just created.
3. In Eclipse, right-click on your project, select "Build Path" and then "Configure Build Path". Click the "Libraries" tab, then "Add External JARs" and find the binary CC file that you just placed in the /lib folder.
To add JavaDoc and API source code:
1. Open ComputerCraft1.63.jar with any 7zip or any other archivator, go to "api", there will be 2 folders: "doc" and "src". The first contains JavaDoc, the second source. Copy these folders to any folder.
2. Create 2 files called "ComputerCraft1.63JavaDoc.zip/jar" and "ComputerCraft1.63APISRC.zip/jar". Into the first file extract "doc" folder, then, open "src" folder, there you'll see "dan200" folder", add it into "ComputerCraft1.63APISRC.zip/jar".
3.Move "ComputerCraft1.63JavaDoc.zip/jar" and "ComputerCraft1.63APISRC.zip/jar" into /lib folder.
4. Open the "Referenced Libraries" tree in your package explorer, find "ComputerCraft1.63.jar" and right-click on it; go to "Properties" -> "External location" -> "External file" and navigate to wherever you stored the source file (ComputerCraft1.63APISRC.zip/jar), preferably right next to the binary in the /lib folder.
5. In Eclipse, right-click on your project, select "Build Path" and then "Configure Build Path". Click the "Libraries" tab, then find ComputerCraft1.63.zip/jar, select "JavaDoc Location" and click "edit" and find the JavaDoc CC file that you just placed in the /lib folder.
6. Run your debug configuration and see if everything is still working; if so, great! If not, you may need to run setupDev and setupDecomp workspace one more time:
a. gradlew setupDevWorkspace
b. gradlew setupDecompWorkspace
c. gradlew eclipse // will vary depending on your IDE
Once you have the debug client working, you should see that both your mod and CC are loaded; if you start a new game, anything added by CC will be in that game as well and should be fully functional.