Add documentation for writing new packages

docs/new_packages.md

+# Creating a Pyodide package
+
+Pyodide includes a set of automatic tools to make it easier to add new
+third-party Python libraries to the build.
+
+These tools automate the following steps for building a package:
+
+- Downloading a source tarball (usually from PyPI)
+- Confirming integrity of the package by comparing it to a checksum
+- Applying patches, if any, to the source distribution
+- Adding extra files, if any, to the source distribution
+- If the package includes C/C++/Cython extensions:
+  - Building the package natively, keeping track of invocations of the native
+    compiler and linker
+  - Rebuilding the package using emscripten to target WebAssembly
+- If the package is pure Python:
+  - Running the `setup.py` script to get the built package
+- Packaging the results into an emscripten virtual filesystem package, which
+  comprises:
+  - A `.data` file containing the file contents of the whole package,
+    concatenated together
+  - A `.js` file which contains metadata about the files and installs them into
+    the virtual filesystem.
+
+Lastly, a `packages.json` file is output containing the dependency tree of all
+packages, so `pyodide.loadPackage` can load a package's dependencies
+automatically.
+
+## The meta.yaml file
+
+Packages are defined by writing a `meta.yaml` file. The format of these files is
+based on the `meta.yaml` files used to build [Conda packages](TODO: Add URL),
+though it is much more limited. The most important limitation is that Pyodide
+assumes there will only be one version of a given library available. Despite the
+limitations, keeping the file format as close as possible to conda's should make
+it easier to use existing conda package definitions as a starting point to
+create Pyodide packages. In general, however, one should not expect Conda
+packages to "just work" with Pyodide. (In the longer term, Pyodide may use conda
+as its packaging system, and this should hopefully ease that transition.)
+
+The supported keys in the `meta.yaml` file are described below.
+
+### `package`
+
+#### `package/name`
+
+The name of the package. It must match the name of the package used when
+expanding the tarball, which is sometimes different from the name of the package
+in the Python namespace when installed. It must also match the name of the
+directory in which the `meta.yaml` file is placed.
+
+#### `package/version`
+
+The version of the package.
+
+### `source`
+
+#### `source/url`
+
+The url of the source tarball.
+
+The tarball may be in any of the formats supported by Python's
+`shutil.unpack_archive`:
+
+  tar, gztar, bztar, xztar, zip
+
+#### `source/md5`
+
+The MD5 checksum of the tarball.  (TODO: More hash types should be supported in the future).
+
+#### `source/patches`
+
+A list of patch files to apply after expanding the tarball. These are applied
+using `patch -p1` from the root of the source tree.
+
+#### `source/extras`
+
+Extra files to add to the source tree. This should be a list where each entry is
+a pair of the form `(src, dst)`. The `src` path is relative to the directory in
+which the `meta.yaml` file resides. The `dst` path is relative to the root of
+source tree (the expanded tarball).
+
+### `build`
+
+#### `build/cflags`
+
+Extra arguments to pass to the compiler when building for WebAssembly.
+
+#### `build/ldflags`
+
+Extra arguments to pass to the linker when building for WebAssembly.
+
+#### `build/post`
+
+Shell commands to run after building the library. These are run inside of
+`bash`, and there are two special environment variables defined:
+
+- `$BUILD`: The root of the built package. (`build/lib.XXX/` inside of the
+  source directory). This is what will be installed into Python site-packages.
+- `$PKGDIR`: The directory in which the `meta.yaml` file resides.
+
+(This key is not part of the conda package specification).
+
+### `requirements`
+
+#### `requirements/run`
+
+A list of required packages.
+
+(Unlike conda, this only supports package names, not versions).