readme: document gapps/microg flags and add build commands

README.mkdn

@@ -8,18 +8,41 @@ To get started with Android/AmogOS, you'll need to get
 familiar with [Repo](https://source.android.com/source/using-repo.html) and [Version Control with Git](https://source.android.com/source/version-control.html).
 
 To initialize your local repository using the LMODroid trees, use a command like this:
-```
+```bash
 repo init -u https://github.com/AmogOS-Rom/android_manifest/ -b thirteen --git-lfs
 ```
+
 Then to sync up:
-```
+```bash
 repo sync -c -j$(nproc --all) --force-sync --no-clone-bundle --no-tags
 ```
+
+Before building you can set environment variables to include gapps or microg.
+```bash
+# include gapps in the build
+export GAPPS=true
+
+# include microg
+export GAPPS=false
+
+# unset env variables for vanilla, if previously set
+unset GAPPS
+```
+
+To start the build you should run the following commands.
+```bash
+source build/envsetup.sh
+
+# replace $codename with the actual codename of your device
+lunch lmo_$codename-userdebug
+
+m bacon
+```
+
 Credits
 ===========
 
 - LmoDroid
 - LeafOS
 - Radek Błędowski
 - Игорь Ч (Igor121)
-