docs: Document custom installation (#11640)

* :memo: Document custom installation

* :heart: Implement feedback

* :wrench: Fix linting errors

docs/tutorial/installation.md

@@ -10,7 +10,7 @@ app:
 npm install electron --save-dev
 ```
 
-See the 
+See the
 [Electron versioning doc](electron-versioning.md)
 for info on how to manage Electron versions in your apps.
 
@@ -32,7 +32,7 @@ If you want to change the architecture that is downloaded (e.g., `ia32` on an
 npm install --arch=ia32 electron
 ```
 
-In addition to changing the architecture, you can also specify the platform 
+In addition to changing the architecture, you can also specify the platform
 (e.g., `win32`, `linux`, etc.) using the `--platform` flag:
 
 ```shell
@@ -43,22 +43,78 @@ npm install --platform=win32 electron
 
 If you need to use an HTTP proxy you can [set these environment variables](https://github.com/request/request/tree/f0c4ec061141051988d1216c24936ad2e7d5c45d#controlling-proxy-behaviour-using-environment-variables).
 
+## Custom Mirrors and Caches
+During installation, the `electron` module will call out to [`electron-download`](https://github.com/electron-userland/electron-download) to download prebuilt
+binaries of Electron for your platform. It will do so by contacting GitHub's
+release download page (`https://github.com/electron/electron/releases/tag/v$VERSION`,
+where `$VERSION` is the exact version of Electron).
+
+If you are unable to access GitHub or you need to provide a custom build, you
+can do so by either providing a mirror or an existing cache directory.
+
+#### Mirror
+You can use environment variables to override the base URL, the path at which to
+look for Electron binaries, and the binary filename. The url used by `electron-download`
+is composed as follows:
+
+```txt
+url = ELECTRON_MIRROR + ELECTRON_CUSTOM_DIR + '/' + ELECTRON_CUSTOM_FILENAME
+```
+
+For instance, to use the China mirror:
+
+```txt
+ELECTRON_MIRROR="https://npm.taobao.org/mirrors/electron/"
+```
+
+#### Cache
+Alternatively, you can override the local cache. `electron-download` will cache
+downloaded binaries in a local directory to not stress your network. You can use
+that cache folder to provide custom builds of Electron or to avoid making contact
+with the network at all.
+
+* Linux: `$XDG_CACHE_HOME` or `~/.cache/electron/`
+* MacOS: `~/Library/Caches/electron/`
+* Windows: `$LOCALAPPDATA/electron/Cache` or `~/AppData/Local/electron/Cache/`
+
+On environments that have been using older versions of Electron, you might find the
+cache also in `~/.electron`.
+
+You can also override the local cache location by providing a `ELECTRON_CACHE`
+environment variable.
+
+The cache contains the version's official zip file as well as a checksum, stored as
+a text file. A typical cache might look like this:
+
+```sh
+├── electron-v1.7.9-darwin-x64.zip
+├── electron-v1.8.1-darwin-x64.zip
+├── electron-v1.8.2-beta.1-darwin-x64.zip
+├── electron-v1.8.2-beta.2-darwin-x64.zip
+├── electron-v1.8.2-beta.3-darwin-x64.zip
+├── SHASUMS256.txt-1.7.9
+├── SHASUMS256.txt-1.8.1
+├── SHASUMS256.txt-1.8.2-beta.1
+├── SHASUMS256.txt-1.8.2-beta.2
+├── SHASUMS256.txt-1.8.2-beta.3
+```
+
 ## Troubleshooting
 
-When running `npm install electron`, some users occasionally encounter 
+When running `npm install electron`, some users occasionally encounter
 installation errors.
 
-In almost all cases, these errors are the result of network problems and not 
-actual issues with the `electron` npm package. Errors like `ELIFECYCLE`, 
-`EAI_AGAIN`, `ECONNRESET`, and `ETIMEDOUT` are all indications of such 
-network problems. The best resolution is to try switching networks, or 
+In almost all cases, these errors are the result of network problems and not
+actual issues with the `electron` npm package. Errors like `ELIFECYCLE`,
+`EAI_AGAIN`, `ECONNRESET`, and `ETIMEDOUT` are all indications of such
+network problems. The best resolution is to try switching networks, or
 just wait a bit and try installing again.
 
-You can also attempt to download Electron directly from 
-[electron/electron/releases](https://github.com/electron/electron/releases) 
+You can also attempt to download Electron directly from
+[electron/electron/releases](https://github.com/electron/electron/releases)
 if installing via `npm` is failing.
 
-If installation fails with an `EACCESS` error you may need to 
+If installation fails with an `EACCESS` error you may need to
 [fix your npm permissions](https://docs.npmjs.com/getting-started/fixing-npm-permissions).
 
 If the above error persists, the [unsafe-perm](https://docs.npmjs.com/misc/config#unsafe-perm) flag may need to be set to true: