E.g. of invoice for conference w/ split IncomeType

An example of an invoice for a conference where the income type is split
between RBI and Donations.  Include a ledger reporting example that
shows the total of donations, and explain in the documentation why
that's useful.

accounts/books.ledger

@@ -3,3 +3,4 @@
 
 !include config/config-npo.ledger
 !include org/main-org.ledger
+!include fooproject/foo.ledger

accounts/config/config-tags.ledger

@@ -46,7 +46,7 @@ tag IncomeType
 
 tag Program
     assert value =~ /[ A-z0-9\-]+(:[ A-z0-9\-]+)*/
-    check  value =~ /^(Main Org:(Overhead|Direct Fundraising|Software:Internal))$/
+    check  value =~ /^((Foo|Main Org):(Overhead|Direct Fundraising|Software:Internal|Conference.*))$/
 
 
 ; Entity tag is just a string, but shouldn't have spaces, just letters,

accounts/org/main-org.ledger

@@ -41,4 +41,3 @@
         ;Invoice: accounts/documentation/org/expenses/developer/2012-07-05_developer-invoice.txt
     Asset:Checking                    $-5,000.00
         ;TaxImplication: 1099
-

npo-ledger-cli-tutorial.md

@@ -455,6 +455,7 @@ Assume for this example that the shell variables `entity`, `program`, and
 `invoice` are set as follows:
 
     $ entity=Sir-Moneybags; program='Main.*Org:.*Direct'; invoice=2012-05-30
+
 If the invoice was paid, this ledger command will have two lines of output,
 and the second line will be a transaction on the payment date.  If only one
 line appears, it's the receivable accrual and we see the invoice is not paid.
@@ -465,6 +466,20 @@ Alternatively, the following command will only have output if the invoice is unp
 
     $ ledger -f accounts/books.ledger  -V --limit 'tag("Entity") =~ /'$entity'/ and tag("Program") =~ /'$program'/ and tag("Invoice") =~ /'$invoice'/' bal /Accrued/
 
+### Calculating Donation Portion of Invoice
+
+As described in
+[IRS 501(c)(3) rules](https://www.irs.gov/charities-non-profits/substantiating-charitable-contributions),
+a charity must substantiate portions of a contribution that are RBI, and
+portions that are donations separately and inform the donor which portion was
+a donation that may be eligible for tax deduction.  Use of the `IncomeType`
+tag facilities collection of the necessary data to determine the total, and
+the following command line will total up all the donation portions for a
+particular invoice:
+
+    $ entity=BigCorp; program=Foo.*Conf.*2017; invoice=2016-01-30
+    $ ledger -f accounts/books.ledger  -V --sort d --limit 'tag("IncomeType") =~ /Donation/ and tag("Entity") =~ /'$entity'/ and tag("Program") =~ /'$program'/ and tag("Invoice") =~ /'$invoice'/' reg
+
 Analysis of the Data
 --------------------