Add development instructions

DEV.md

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+# Setting up a development environment
+
+## Preprequisites
+
+- NodeJS & npm
+- VSCode
+- (optional) A clean Python venv
+
+## Building slither-lsp
+
+After cloning slither-lsp in a convenient location, run the following command to install it and its dependencies to your Python environment:
+
+    pip install .
+
+You may then want to actually _uninstall_ it afterwards, so that any change made to the source code is directly picked up the next time you execute the module without having to reinstall it.
+
+    pip uninstall slither-lsp
+
+## Starting slither-lsp in TCP mode
+
+In order to make it easier (or even possible) to attach a debugger to the Python process that runs slither-lsp, it's possible to start it in TCP mode:
+
+    python -m slither_lsp tcp --port 12345
+
+## Building slither-vscode
+
+After cloning slither-vscode in a convenient location, run the following commands from the source directory:
+
+    npm install
+    npm install -g vsce
+    npm run compile
+
+## Running a VSCode instance with slither-vscode loaded
+
+    env EXISTING_LANGUAGE_SERVER_PORT=12345 code --extensionDevelopmentPath=/path/to/slither-vscode
+
+## Debugging slither-lsp and slither-vscode at the same time from the same VSCode instance
+
+This is the `launch.json` file I personally use, you will need to adjust paths accordingly:
+
+    {
+        "version": "0.2.0",
+        "configurations": [
+            {
+                // This configuration launches slither-lsp as a subprocess and needs it to be installed
+                "name": "Extension",
+                "type": "extensionHost",
+                "request": "launch",
+                "runtimeExecutable": "${execPath}",
+                "args": [
+                    "--extensionDevelopmentPath=${workspaceFolder}/slither-vscode"
+                ],
+                "outFiles": [
+                    "${workspaceFolder}/slither-vscode/out/**/*.js"
+                ],
+                "skipFiles": [
+                    "**/app/out/vs/**.js"
+                ],
+                "env": {
+                    "PATH": "${env:PATH}:${workspaceFolder}/slither-venv/bin"
+                }
+            },
+            {
+                "name": "Extension (attach to existing over network)",
+                "type": "extensionHost",
+                "request": "launch",
+                "runtimeExecutable": "${execPath}",
+                "args": [
+                    "--extensionDevelopmentPath=${workspaceFolder}/slither-vscode"
+                ],
+                "outFiles": [
+                    "${workspaceFolder}/slither-vscode/out/**/*.js"
+                ],
+                "skipFiles": [
+                    "**/app/out/vs/**.js"
+                ],
+                "env": {
+                    "EXISTING_LANGUAGE_SERVER_PORT": "12345"
+                }
+            },
+            {
+                "name": "LSP Server",
+                "type": "debugpy",
+                "request": "launch",
+                "module": "slither_lsp",
+                "cwd": "${workspaceFolder}/slither-lsp",
+                "python": "${workspaceFolder}/slither-venv/bin/python3",
+                "args": [
+                    "--loglevel=INFO",
+                    "tcp",
+                    "--port=12345"
+                ]
+            }
+        ]
+    }