📝 add build documentation

🔧 explicitly include appveyer config to make it easier to fire up under another account

Author: fluffynuts

BUILDING.md

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+## Building log4net
+
+Log4net provides support for a wide array of targets, including
+- older .net 2 and 3.5 (including client profiles)
+- more modern net40/net45
+- netstandard1.3/2.0
+
+As such, it does require a relatively full .net environment on Windowsa to build.
+
+Options:
+- build locally. Suggested environment:
+    - Pre-requisites:
+        - Visual Studio 2019 Build Tools
+        - Ensure you have .NET Framework 3.5 SP1 installed
+            - on Win10+, this can only be installed via Add/Remove Windows Components
+            - on other platforms, see https://dotnet.microsoft.com/download/dotnet-framework/net35-sp1
+            - Building against .net 2/3.5, especially Client Profile, is not supported on Mono
+    - Binaries can be built with a Visual Studio or Rider installation
+    - Binaries, packages and a release zip can be built via commandline
+        - Ensure that you have a reasonably modern NodeJS installed (at least version 8+)
+            - `npm ci`
+            - `npm run build`
+            - optionally `npm test` to run all tests
+            - optionally `npm run release` to generate release artifacts
+- build via the vs2019 Windows AppVeyer image. There is an appveyer.yml file
+    included which (should) build if you set up AppVeyer to track
+    your fork. AppVeyer is free for open-source projects.
+    (TODO: should have a link to the official AppVeyer build)
+- build via the CircleCI Windows image. There is included configuration for CircleCI.
+    CicleCI is free (with usage limitations) for open-source projects.

appveyer.yml

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+version: 1.0.{build}
+image: Visual Studio 2019
+build_script:
+- cmd: >-
+    npm ci
+    npm test
+    npm run release
+
+artifacts:
+- path: build/artifacts/*