Added manual building instructions

BUILDING.md

@@ -1,15 +1,17 @@
 # Building
 
-Building session binaries is done using github actions. Windows and linux binaries will build right out of the box but there are some extra steps needed for Mac OS
+## Automated
 
-## Mac OS
+Automatic building of session binaries is done using github actions. Windows and linux binaries will build right out of the box but there are some extra steps needed for Mac OS
+
+### Mac OS
 
 The build script for Mac OS requires you to have a valid `Developer ID Application` certificate. Without this the build script cannot sign and notarize the mac binary which is needed for Catalina 10.15 and above.
 If you would like to disable this then comment out `"afterSign": "build/notarize.js",` in package.json.
 
 You will also need an [App-specific password](https://support.apple.com/en-al/HT204397) for the apple account you wish to notarize with
 
-### Setup
+#### Setup
 
 Once you have your `Developer ID Application` you need to export it into a `.p12` file. Keep a note of the password used to encrypt this file as it will be needed later.
 
@@ -40,3 +42,56 @@ base64 -i certificate.p12 -o encoded.txt
     5.  Team ID (Optional)
         * Name: `SIGNING_TEAM_ID`
         * Value: The apple team id if you're sigining the application for a team
+
+## Manual
+
+### Node version
+
+You will need node `10.13.0`.
+This can be done by using [nvm](https://github.com/nvm-sh/nvm) and running `nvm use` or you can install it manually.
+
+### Prerequisites
+
+<details>
+<summary>Windows</summary>
+
+Building on windows should work straight out of the box, but if it fails then you will need to run the following:
+
+```
+npm install --global --production windows-build-tools@4.0.0
+npm install --global node-gyp@latest
+npm config set python python2.7
+npm config set msvs_version 2015
+```
+
+</details>
+
+<details>
+<summary>Mac</summary>
+
+If you are going to distribute the binary then make sure you have a `Developer ID Application` certificate in your keychain.
+
+You will then need to generate an [app specific password](https://support.apple.com/HT204397) for your Apple ID.
+
+Then run the following to export the variables
+
+```
+export SIGNING_APPLE_ID=<your apple id>
+export SIGNING_APP_PASSWORD=<your app specific password>
+export SIGNING_TEAM_ID=<your team id if applicable>
+```
+
+</details>
+
+### Commands
+
+Run the following to build the binaries for your specific system OS.
+
+```
+npm install yarn --no-save
+yarn install --frozen-lockfile
+yarn generate
+yarn build-release
+```
+
+The binaries will be placed inside the `release/` folder.