PR preview docs (#105)

* PR preview docs

* add link to remind to clean up this code once less evil way is possible

* Update docs/make.jl

* fix typo

* .

* .

* fix docs badge links

* remove debug println

* =highlight readme

docs/make.jl

@@ -14,4 +14,24 @@ makedocs(
     ],
 )
 
-deploydocs(repo="github.com/JuliaDiff/ChainRules.jl.git")
+const repo="github.com/JuliaDiff/ChainRules.jl.git"
+const PR = get(ENV, "TRAVIS_PULL_REQUEST", "false")
+if PR == "false"
+    # Normal case, only deply docs if merging to master or release tagged
+    deploydocs(repo=repo)
+else
+    @info "Deploying review docs for PR #$PR"
+    # TODO: remove most of this once https://github.com/JuliaDocs/Documenter.jl/issues/1131 is resolved
+
+    # Overwrite Documenter's function for generating the versions.js file
+    foreach(Base.delete_method, methods(Documenter.Writers.HTMLWriter.generate_version_file))
+    Documenter.Writers.HTMLWriter.generate_version_file(_, _) = nothing
+    # Overwrite necessary environment variables to trick Documenter to deploy
+    ENV["TRAVIS_PULL_REQUEST"] = "false"
+    ENV["TRAVIS_BRANCH"] = "master"
+
+    deploydocs(
+        devurl="preview-PR$(PR)",
+        repo=repo,
+    )
+end