ci update, cpp update

README.md

@@ -1,3 +1,42 @@
 # wallet2_api.h (but this time C compatible)
 
-Wrapper around wallet2_api.h that can be called using C api.
+Wrapper around wallet2_api.h that can be called using C api.
+
+## Contributing
+
+To contribute you can visit git.mrcyjanek.net/mrcyjanek/monero_c and open a PR, alternatively use any other code mirror or send patches directly.
+
+## Building (android)
+
+Builds are provided in the [release tab](https://git.mrcyjanek.net/mrcyjanek/monero_c/releases), built using Gitea Runners. Building locally is possible as well, althought it is rather a heavy task which takes ~2 hours to finish (excluding enviroment setup and some downloads).
+
+Base image for the runner is `registry.mrcyjanek.net/androidndk:r17c`, which contains preinstalled NDK. `Dockerfile` can be obtained from [mrcyjanek/CIimages](https://git.mrcyjanek.net/mrcyjanek/CIimages/src/branch/master/Dockerfile.androidndk-r17c) repository.
+
+Then to build `.github/workflows/*.yml` files are used.
+
+Local build?
+
+```bash
+$ act --pull=false -Pandroidndk-r17c=registry.mrcyjanek.net/androidndk:r17c
+```
+
+For development?
+
+```bash
+$ timeout 5 act --pull=false -Pandroidndk-r17c=registry.mrcyjanek.net/androidndk:r17c # needed to clear cache.
+$ act --pull=false -Pandroidndk-r17c=registry.mrcyjanek.net/androidndk:r17c --reuse
+$ docker ps
+CONTAINER ID IMAGE .....................................
+d0626dcd8c5d registry.mrcyjanek.net/androidndk:r17c ....
+$ docker commit d0626dcd8c5d monero_c:dev
+$ docker run --rm -it \
+    -v $PWD/libbridge:/opt/wspace/libbridge_up \
+    --entrypoint /bin/bash \
+    monero_c:dev
+[docker] $ export 'PATH=/usr/cmake-3.14.6-Linux-x86_64/bin:/opt/android/toolchain/aarch64-linux-android/bin:/opt/android/toolchain/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
+[docker] $ cd /opt/wspace/libbridge_up
+[docker] $ rm -rf build && mkdir build && cd build
+[docker] $ env CC=clang CXX=clang++ cmake -DANDROID_ABI=-arm64-v8a ..
+[docker] $ make
+# Resulting file will be available in the current directory.
+```